Not quite sure whats causing it, but when I'm playing as a fairy, after I take damage for the first time I will get the death screen, but hitting respawn just clears the death screen and I can continue as normal without actually respawning. Then my character model is bugged out in the death animation until I die for real.
(Just tested this after I typed this using a separate fresh install and after consuming the fairy dew, I went to a desert and touched a cactus and the bug occurred as I stated. Only mods I have installed while testing are this mod and JEI)

Edit: 1. The bug occurs with all forms of fairy transformation, not just the fairy dew.
2. The bug does not occur with dwarf or titan transformation
3. it looks like after multiple "fake death" occurrences that the bug happens a lot less, but still happens occasionally, as of now I cant find any consistency in making it happen

Unless you're using ArtemisLib for another mod, I recommend not using it with T&B, I helped work on that a while back, but I believe the owner has abandoned it, I could theoretically work on it, but I don't have the time, and it does cause some issues related to height/scaling.
That might be the issue with the climbing stairs, if not, let me know.
The Health thing happens because I modify the health, the game sees you as suddenly losing a bunch of hp, and assumes it's damage, so it attempts to lower your health, but because the max health is reduced, it thinks it killed you, That seems to be an issue with how Vanilla MC handles health.
I let ArtemisLib handle the height and most things related to scaling when it's installed, You don't necessarily have to remove it, but I do recommend at least turning the compatibility for it off in the T&B config.

I am having this same issue in 31.4. Put the fairy ring on, "You Died" then:

Only mods installed:
ArtimesLib
Baubles
Trinkets

Unfortunately, this has been a thorn in my side for a long time, I know why it happens, but I haven't figured out a good way to prevent it from happening, had a few people in my discord mention it still happens even with the latest version, I'll have to try something new when I get the chance, At the moment I'm working on a 1.16 version so I've temporarily stopped development on 1.12, I plan to come back to it when I get a 1.16+ version out.

I'm also having issues climbing stairs after removing the fairy ring, but I'm not sure it is related to the above issue. Killing myself for real fixes both issues.

FYI, I only recently updated from 30.1, which didn't have these issue and worked flawlessly.

I actually installed ArtemisLib only because I was already having issues with Trinkets. I was hoping that might fix it, no luck.

ah, ok, Uhm, The Health Thing I Definitely have to work on.
The Failing to Climb Stairs sounds like the stepheight isn't properly resetting on death, you can disable the Step Height modifications in the Fairy Race settings to see if that fixes the climbing stairs issue

Thank you for Reporting, it Really Helps.
I'm already aware of the issue and I'm Currently working on it, Someone from my discord has been helping me test, and it's apparently happening in other mods that add things that Reduce the players health.
So it might be a Vanilla Bug.
That being said, I'm still trying to find a solution, and if I can't solve it when lowering HP, I'm probably going to switch from lowering HP, to probably increasing the damage the player takes when a Fairy instead.

@florabean Good News, I Managed to Solve the issue with the Fake death, as long as nothing else crops up, I'm Hoping to push a 0.30.1 with the fix sometime within the week

nice! lookin forward to it

0.30.1 Released and the Fake Death Issue should be fixed, but let me know if it still happens @florabean