This issue happens occasionally, as I have no way to make sure it could be replicated.
Maybe could be replicated by having a laggy connection to the server, and having a screen which have a URL and not turned off. Note that there is even no way to turn off a screen than breaking it?
And also, duplicate browsers for the same screen might be created occasionally, one seems to be created when joining server (before joining world), and another is created after joining world.

MC 1.20.1, Forge 47.3.12, WebDisplays 2.0.1, MCEF 2.1.6
maybe a duplicate to #65