The timeout is 30 seconds, it takes me less than 3 seconds to try to paste the copied content.
As per your suggestion I increased the timeout to 10 minutes (600 seconds) to no avail.
I however found out what the problem is, Enpass uses two methods for pasting:
Wheel click (clipboard is not populated from Enpass, content selected previously)
Right click + Paste (clipboard from Enpass is used)
In other words Wheel click and Right click have different content when Enpass is used (this is the only application to do so from my experience with Ubuntu). If abc is copied from a text document, and xyz is copied form Enpass by clicking on Copy icon, there are two clipboards, depending which buttons are pressed.
I tested the two clicks/clipboards in Terminator, Terminal, Text Editor and few other apps, the behaviour is the same, i.e. only Enpass seems to use two clipboards.
Whilst I can live with the workaround, could you see on your side if this is the case?