Thanks for your bug report. As far as I can tell, this is behaving as expected.
mIRC added support for DALnet's numerics 650 and 651 in 2002.
When mIRC sees numeric 650, it assumes that one or more service messages will follow, so it plays a single notice event sound, displays the message "Services Message" in reverse (as requested by users at the time), and then displays a list of incoming services messages. When it receives numeric 651, this indicates the end of the group of related services messages.
That said, it looks like InspIRCd does not send numeric 651, which means that mIRC cannot tell when the services messages group has ended.
I am going to change the implementation of this to be less dependent on receiving numeric 651. This change should be in the next beta.