1) Install mIRC into a clean, empty folder with no DLLs, scripts, etc. using the portable option in the installer.
2) Run mIRC, maximize the status window, connect to a server, join two channels.
3) Run Photoshop. Do you see the same issue?

Yes. The most interesting thing is that I even installed a newer version of "Photoshop 2020" and this bug has not gone anywhere.

If you follow the above steps and run other applications, other than Photoshop, can you reproduce the issue?

No. Apparently, this bug, which I see, is solely related to the run of the "Photoshop" program. It's funny.

Well, if other users could not detect this bug in the new version of the mIRC client 7.65+, then will have to wait for someone to find bug and will leave (write) your own his a report about it here.
Perhaps this bug will manifest itself in the future due to something else. In addition, probably not everyone has migrated to the most recent version of the client yet or vice versa rolled back to the old version.

Let's wait...
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