@Khaled Darn! I thought we had a reproducible issue.
I am running mIRC 7.52 not the beta. I will try the beta later, but I don't see any changes in the change log that would make a difference.
I suppose it could be variations either in Windows 10 version or graphics chip / drivers.
What version of Windows 10 are you running? Mine is 1709.
What graphics are you using? Mine is Intel HR Graphics 4600 on latest drivers from Feb 2018 v184.108.40.20663.