This isn't really a bug of mIRC, but mainly the DALnet server.
If you find a channel that is set to "Sticky Topics", go into that channel [if it isn't restricted and you're not akicked, otherwise access doesn't matter] type in the text-bar "//hop | //topic #somechannel blah blah", and copy it (without quotes).
This will cycle you through the channel and back, when it does that, quickly paste the "//hop | //topic #somechannel blah blah" and do it again. This allows you to change the topic in a channel with "Sticky Topics" even if you aren't a SOp/AOp/Founder.