I hope I'm not muddying the waters but this has been driving me crazy also. I am using Firefox on Windows 10. Enpass Beta 6.7.0(865). Firefox 91.0.2(x64)
Most times if I click on the Enpass icon, whether it autoshows any relevant entries or whether I search, when I dblclick the relevant entry the popup closes but nothing else happens. I then repeat the process and it works fine autofilling the page. It also fails if instead of dblclicking, I use the keyboard, pressing arrow keys down to relevant entry and press Enter. It has not always been like this. It has been maybe a couple of months. I do not know whether the problem started with a change in Enpass,Firefox or Windows as it was some time before I decided there really was a problem.