I can't remember how long I've had this issue! I just realized today that it was due to my Bold Text setting being turned on. I thought it was issues with a few of the apps I use. Hopefully it'll be fixed eventually

I solved this by setting only the first text field's textContentType field to .newPassword. I set the second text field's textContentType field to just .password. iOS autofill is smart enough to suggest and pre-populate the strong password for both secure text fields, but if choosing your own password, selecting the second text field doesn't force the strong password suggestion twice, causing the loop. Hope this helps!