Thanks for your bug report. This is due to the way font-linking works. If you are using a font that cannot display a string of characters, mIRC asks Windows for a font that can optimally display all of them and then alternates between fonts where necessary when rendering the text. mIRC caches the fonts it is using while running. At some point, Windows may decide that a different font is optimal and will tell mIRC to use that font instead. If you exit mIRC and run it again, the cache is built again for that session, so the fonts used could be different again.
mIRC also bitmap caches some displayed text for speed, which is updated in certain contexts, which may be what you are seeing. mIRC clears this cache when fonts change, however it sounds like it is not doing it in the specific situation you are seeing. If you can provide a step by step method that reproduces this, I will try to reproduce/fix it.
