Thanks for your bug report. I have not been able to reproduce this yet. Here are the steps I am following:

1) Run mIRC
2) In the status Windows, press the AltGr key (the right Alt key), and then press the letter "e".

If I hold down the letter "e", it generates an accented "é" repeatedly.
If I let go of the letter "e", and then press it again, it generates an "é" again.
If I let go of both the letter "e" and the AltGr key, and then press them again, it generates an "é".

Which language Windows/keyboard are you using?

If you restart your computer, does the problem go away?