Just putting this here for the record. I wondered why pets would sometimes stop attacking midway through a fight and didn't know if it was an issue with the server code and pet hold or something. It turns out it's to do with petassist=1 in the mq2melee ini file for the character. This setting is supposed to stop your pet from engaging unless the mob is within petrange distance (petrange=75 is the default).
If you have pet hold then this isn't necessary anyway so just set petassist=0 to solve this issue. I'm sure some of you already worked this out but it saves me hunting down potential bugs in the server code
Not sure why mq2 does that maybe it's a bug.