Alright so, in the rules for Checks, under Step 4: Degrees of Success (Player Core 1, Page 401) it states:
"Some actions and abilities have stronger effects on a critical success or failure. For example, a Strike deals double damage on a critical hit. If an effect doesn’t list a critical success effect, the critical success effect is the same as the success effect, and the same goes for critical failures."
How does this interact with abilities that add effects that an action doesn't have?
Lets take for example Certain Strike or Confident Finisher.
Both of those add a Failure effect to Strike, which doesn't have one. Per the rules cited before in checks, this added effect would also apply on a Critical Failure, since Strike doesn't list a Critical Failure effect and thus we use the (newly added) Failure effect, right?
This is how I've always ruled it, but I recently did a one-shot with some new players and they expressed confusion when I allowed the Swashbuckler to deal their Confident Finisher Failure damage on a Natural 1, saying their other GMs hadn't ruled that way.
Have I been running things wrong this entire time or is this just a hidden quirk of the rules people might (rightfully) miss?
Well.. you are correct in that failure effects also apply on critical failure if nothing is listed.
However you have still been running it wrong as you missed the relevant text within Swashbuckler finishers and the press trait.
Some finisher actions also grant an effect on a failure. Effects added on a failure don't apply on a critical failure.Some actions with the press trait also grant an effect on a failure. The effects that are added on a failure don’t apply on a critical failure.
Ohhh thank you very much! So much stuff to keep track of in this game, I swear.