Hello friend.

Double projectile doesn't apply for everything and was always the case.
Autoturret I think it just doubles the fire rate (you don't see visually a second projectile)
Gunslinger/Deathgazer/Evasion takes projectile damage bonuses but never intended to have a double projectile.

Blistering Embrace, it's written "on hit" now but before Bloodtrail it was "when hitting an annemy with an Attack" so pretty sure it only procs with auto attack but I can be wrong.

Have fun

If this doesn't apply to certains skills then they should remove the "Projectile" tag from these or just specify the skills which are not compatible with Archion's Teaching. Tear of Etheliel is not compatible too while to me it should absolutely be. Doubling this on a skill with a cooldown wouldnt break the game.

Also making it work with other passive node triggering a projectile would add more depth to the game. It's still something I don't understand with this game, there is so much potential but everything feels confusing because you're not sure if it's working or not...
There is also a passive called Residual Energy in the Warlock branch but there is no info if it's triggering or not, just like Ancient Fervour which provide a buff you can see above your hp.

They should REALLY work on this because you can't really experiment with builds if it's not working as expected.
That was an issue in the original game and it's still here after introducing this chronicle. Just like some other bugs...

I'm still faithful about this game but they need to stop working on things that are not essential to the game, they are shooting themselves in the foot.
They added new skins, a pet (useless tbh) system, a new useless skill, etc...