Okay, I have tried the following:
1) Run mIRC.
2) In Status Window 1 - Connect to ZNC using /server zncaddress zncnick:zncpass -i nick
3) In Status Window 2 - Connect to same server using /server serveraddress -i nick
4) In Status Window 1 - open a query window 1 to nick
5) In Status Window 2 - open a query window 2 to zncnick
6) In Status Window 1 - disconnnect
7) In Status Window 1 - connect
When I follow the above steps exactly, the query window in Status Window 1 is renamed from "nick" to "zncnick". That is because, on connect, ZNC is sending a NICK change to set your zncnick:
<- @time=2022-05-05T07:41:42.826Z :firstname.lastname@example.org NICK :zncnick
And in mIRC's nick change handler, if it notices that a user has changed nicks, and you have a query window open with them, it changes the query window target to their new nick so that you can keep talking to them.
Is this the issue you are seeing?