I installed the 24H2 26100.712 pre-release version of Windows 11 and could not reproduce the above issue.
Since this is a beta version of Windows, it is very likely that the cause of the reported issue is that eg. a graphics driver is not working/is still being updated to work correctly with 24H2.
I have, in the past, spent a fair bit of time trying to track down mIRC issues in pre-release versions of Windows only to find that they were due to a Windows/driver bug/issue that was eventually resolved, so at this point I am going to wait until closer to this Windows 11 release date before testing it again.