Hmm...it's not a strange issue, that sounds how the system works OOB? What kind of behaviour did you expect?
If you want to alter it, please format control "IM.close.incident" and check its "Calculation" tab. Find a following calculation:
calculation: if (status in $file="resolved") then (assignment in $file=nullsub(open.group in $file, $lo.home.assignment))
You've to set value in update to "false" to prohibit calculation to change IM ticket assignment group changes.
And if you want just to change the group which receives resolved tickets, please see System Adm. > Ongoing Mainten. > Environment Recs > Service Desk Environment and change Service Desk Group to suite your taste.