I can confirm that Enpass is clearing any copied content - even when it's locked and content is totally not related with Enpass (e.g. copied from kwrite). It cleans both text and image entries.
I'm using recent kubuntu:
It's kubuntu desktop environment, I'm using klipper (not sure if it have any impact but may be worth mentioning).
When I disable auto clibpboard cleanup feature in Enpass clipboard works fine.
Also, I see that Enpass is reacting to clipboard changes - I've copied some text from kwrite while running strace on Enpass process:
First two lines app