Why are saves not transferable back and forth?

I went back to try and upload another one of my characters from PS4 and I keep getting Save game download failed error. I imagine it’s because it says it’s uploading bank and profile info too or something, so now that my PS5 profile which I’ve played for abit doesn’t match what was on my PS4 profile, the save download is failing.

That wouldn’t be an issue if we could transfer back and forth, that way my updated profile and bank on PS5 could be updated on PS4 as well. That’s just a theory though.

Anyone else have this issue and find a way around it?