Casting the spell makes you automatically undetected...so why does it let you make a Hide check with a bonus? What's the point? You're already undetected.
Because the spell only makes you undetected for the short period of time the spell is active, while you are casting it. As soon as the casting effect ends, you are immediately observed again or hidden, based upon the result of your check.
There are a couple of things.
One, as noted previously, the Undetected condition from the invisibility of the spell ends at the end of the spell. You want to be able to remain Undetected afterwards due to Hide, and the spell gives you the action compression to allow for that.
Another is that the invisibility of the spell only provides Undetected on condition that your opponents are relying on vision as their precise sense and don't have other senses that could track your movements. A vague sense (like standard Human smell) wouldn't do much. I don't think that an imprecise sense (like standard Human hearing) also wouldn't work since it mechanically requires action to Seek in order to detect an Undetected creature with an imprecise sense. But there may be creatures with either another precise sense, or an imprecise sense that allows for tracking creatures as a reaction.
So if you do happen to be up against one of these creatures with extra senses, you would still be allowed to Hide at the end of your Stride as long as you have the Cover or Concealment (to those extra senses) necessary.
Yeah, I would recommend allowing the full range of outcomes of the Hide check. And if the invisibility does cause Undetected during the Stride, then the enemy wouldn't know what square you went to, so a successful Hide check while Undetected should allow you to stay Undetected. The spell really shouldn't be overriding that option.
It is a pre remastered spell. I feel like maybe some of the language around hiding and undetected have changed?
From what I remember, the wording changed a bit, but the mechanics didn't.
For example:
A creature that’s hidden is only barely perceptible. You know what space a hidden creature occupies, but little else. Perhaps the creature just moved behind cover and successfully used the Hide action. Your target might be in a deep fogbank or behind a waterfall, where you can see some movement but can’t determine an exact location. Maybe you’ve been blinded or the creature is under the effects of invisibility, but you used the Seek basic action to determine its general location based on hearing alone. Regardless of the specifics, you’re flat-footed to a hidden creature.
When targeting a hidden creature, before you roll to determine your effect, you must attempt a DC 11 flat check. If you fail, you don’t affect the creature, though the actions you used are still expended—as well as any spell slots, costs, and other resources. You remain flat-footed to the creature, whether you successfully target it or not.
A creature that's hidden is only barely perceptible. You know what space it occupies, but little else. Perhaps the creature just moved behind cover and successfully used the Hide action. Your target might be behind a waterfall, where you can see some movement but can't determine an exact location. Maybe you've been blinded or the creature is invisible, but you used the Seek basic action to determine its general location based on hearing alone. Regardless of the specifics, you're off-guard to a hidden creature.
When targeting a hidden creature, before you roll to determine your effect, you must attempt a DC 11 flat check. If you fail, you don't affect the creature, though the actions you used are still expended—as well as any spell slots, costs, and other resources. You remain off-guard to the creature, whether you successfully target it or not.
And:
You huddle behind cover or greater cover or deeper into concealment to become hidden, rather than observed. The GM rolls your Stealth check in secret and compares the result to the Perception DC of each creature you’re observed by but that you have cover or greater cover against or are concealed from. You gain the circumstance bonus from cover or greater cover to your check.
Success If the creature could see you, you’re now hidden from it instead of observed. If you were hidden from or undetected by the creature, you retain that condition. If you successfully become hidden to a creature but then cease to have cover or greater cover against it or be concealed from it, you become observed again. You cease being hidden if you do anything except Hide, Sneak, or Step.
You huddle behind cover or greater cover or deeper into concealment to become hidden, rather than observed. The GM rolls your Stealth check in secret and compares the result to the Perception DC of each creature you're observed by but that you have cover or greater cover against or are concealed from. You get a +2 circumstance bonus to your check if you have standard cover (or +4 from greater cover).
Success If the creature could see you, you're now hidden from it instead of observed. If you were hidden from or undetected by the creature, you retain that condition.
If you successfully become hidden to a creature but then cease to have cover or greater cover against it or be concealed from it, you become observed again. You cease being hidden if you do anything except Hide, Sneak, or Step. If you attempt to Strike a creature, the creature remains off-guard against that attack, and you then become observed. If you do anything else, you become observed just before you act unless the GM determines otherwise. The GM might allow you to perform a particularly unobtrusive action without being noticed, possibly requiring another Stealth check.
If a creature uses Seek to make you observed by it, you must successfully Hide to become hidden from it again.