If my Con mod goes from +2 to +3 at 10th level, do I get 9 extra hp along with what my class hp and Con mod grant me specifically at 10th level? If my Int mod goes up, do I get another training in a skill? Those answers are probably supposed to be "yes", but I can't find where this edition says so; i.e., I only think so because of my experience with P1E, which isn't a given for a player new to the hobby.
For that matter, what about the retroactive benefits of improving a proficiency? For example, the Ranger Snare Savant feat requires me to have at least expert proficiency in Crafting and grants me 3 formulas for snares, plus an additional 3 formulas for every time my proficiency improves. But let's say that for whatever reason, I didn't take Snare Savant until after I already had legendary proficiency in Crafting. How many formulas do I gain, 3 or 9?
Edit: Update 1.3 included an answer to the spoilered above (they still need to change the actual text to say this outright, though since it was an FAQ, hopefully they will do so). The second part still needs to be addressed, though.
You'd gain 9. I see no reason why you shouldn't. The verbage used is clearly to indicate that you get more for each rank without spelling it out.
Oh, I agree you SHOULD. But the language doesn't actually say so. "You gain three formulas for common or uncommon snares plus an additional three for each proficiency rank you have above Expert. Each time your proficiency rank in Crafting improves, you gain three more such formulas." would convey what you and I agree should be the intent. But as it stands, you only gain three formulas for Snare Savant (regardless of proficiency rank), and that number only increases when your rank improves (which cannot happen if you take the feat at legendary).