I have a similar issue in Sequoia 15.0, final release.
The app installed fine, it opens without issues, the browser extensions are working properly (Firefox and Safari), but when the Enpass window is closed (command-W), the application cannot come back to the foreground when called again. It flashes very fast at a rate of 5-10 times a second and the app icon keeps showing up in the dock every time the app tries to come back to the front. This also happens if I option-click on the Desktop to send Enpass to the background.
The only 2 solutions I found are: 1. click on the Enpass icon in the Menu Bar, then click open Enpass. 2. do not close the window app (command-W) but send it to the dock (command-M). It then reopen without a problem like with the command-W method before Sequoia.
Mac Studio M1 Max
macOS Sequoia 15.0 (fresh install)