Here I show a short video of how I can reproduce the accentuation problem. First I press the accent key and then the "E" key ... This repeatedly and this is where mIRC fails a few times.

I installed a portuguese keyboard layout to test this out and it is working fine for me here on Windows 10 in the latest beta.

I press the sticky accent key first, let go of the key, and then I press the letter e. It never fails to produce an accented letter. If I press the keys simultaneously, it is pretty much random whether an accented or non-accented key appears, but this applies to all the applications I have tried.

If you restart your computer and test again, is the issue still present? I mention this because quite a few people have had issues with the AltGr Key in Windows 10.