Female spayed cat urinating out of box; calming treats work for anyone?

First off, this is a behavioral issue. She has seen the vet several times. We have changed litter, boxes are frequently cleaned, they're large enough, etc. She has some possible hip issues, so boxes are low entry.

I want to try a CBD treat to see if it helps. Feliway diffuser hasn't seemed to help either. She lives in a studio apartment with her cat sister and they get along well. She's eating and drinking normally.

Any help would be great!