I have the same gmail and an exchange server for both iPhone and iPad. The phone works like I want and has been for years. I just had the iPad activated to match and when trying to mimic my phone swipes is when I noted the difference.
It appears to only be a google account problem on my iPad. I have my phone setup with same accounts on default Apple mail app. My phone says flag when swi left and that always shows delete.
Well I thought I had it working but it must have just been a few emails that worked correctly. It’s just odd how it functions like I want on the phone but not on the iPad.