As said in the title. im wearing create backtanks both netherite and copper both straight up dont work. 1.20.1 1.0 version

have you tested if they actually do not work, or if it is only the UI that doesn't work?
You should be able to stay underground with a backtank, you'd still see air dropping but once it "depletes" you should be set back to maximum.

i indeed did start taking damage. i thought it was a gui bug at first but guess not.

this happened in the nether btw. i did not test it in the overworld yet. i will do that tomorrow if u want me to

Deleting and redownloading worked