On the Linux version of Enpass, when generating a new password, the "Fill" button re-generates the password when clicking it, before going back to the main view for that entry.
For example, I may be generating a password for an account with a particularly onerous maximum length requirement (which sadly seem to be the norm among many financial and insurance institutions) and wish to use a generation strategy that includes words. In this case I may need to roll through several generated password options until one appears that meets the length requirement I'm targeting. If I then press "fill" to populate the password field on the entry from the password generation pop-up, the password changes as if I had pressed the re-generate button (the looped arrow) button again, and the new password (which could be any length, and I didn't see) is populated into the password field instead of the password I was looking at when I clicked the "Fill" button. I've verified that this behavior is not present in the Windows Store version of Enpass I have installed on another computer.
OS: Pop!OS 20.10 (basically Ubuntu 20.10, installed via apt) 64-bit
Enpass version: 18.104.22.1683
I'm happy to do troubleshooting or additional reproduction steps if necessary.