Thanks for your bug report. If you click on the extra icons, do they do anything? ie. is it just a display glitch?
This may be related to:
29/07/2020 - mIRC v7.62
21.Added workaround for maximized MDI window caption bug in Windows 10
on monitors with some DPI/resolution combinations.
In the next beta, I will disable the above workaround during a remote desktop session but this may re-introduce the Windows 10 display glitch relating to MDI windows that it was intended to fix, so it is a bit of a catch-22 unfortunately.