Most bank apps don't work properly anymore on rooted phones or custom roms.

Sounds like a job for regulations if an essential service like banking can keep users out on such arbitrary terms.

I think only one of the banking apps I use check for rooted phones, and it only uses that to warn the user once on app install and never again.

Regulators can't force banks to provide service in an unsafe environment, which a rooted phone essentially is.

Is it just a warning? Mine explicitly teels me that if anything happens to the bank account they won't be liable because I used the app on a rooted phone.

I didn't have a problem with them since Magisk got so good at hiding itself which has been at least 3 years

s/don't work properly anymore/refuse to run on/

Indeed. They want to correlate the user through their Google account.