This has been occurring for the past year and recent updates haven't improved the situation.
As reported in previous versions Enpass randomly quits and has to be restarted. The app is not "sleeping", it has quit. This becomes apparent when trying to fill a password in Safari and am prompted to start Enpass. Starting Enpass also doesn't connect to the Safari plugin - which is the 2nd bug reported here.
I haven't been able to determine any pattern to the random quitting. Sometimes it occurs in a few hours, other times it's 2-3 days.
Look forward to this being fixed.