This isn't really my purview... But I'm pretty sure when two mons use priority under Trick Room, it defaults to the users speed stat and ignores the effect of Trick Room (as Priority always has). Water Shuriken is is a priority move, as is Shadow Sneak, ergo, Accelgor moves first. Not a bug.
What move did Regice use? Ice Beam should have outsped, so post in the link below about this bug. It may also be related to the tier you are playing in, I am not sure what "Full Potential" means (there could be other modifiers due to the metagame). According to Bulba, the speed "bug" still exists (and I am well aware it is not a reliable source of information). However, I cannot find anything to the contrary and I can't test in Gen 7 myself.
Post here for a proper reply/bug report:
http://www.smogon.com/forums/threads/bug-reports-v2-0-read-op-before-posting.3469932/
EDIT: After doing some digging this isn't even playable on the official server. I would suggest taking it up with the hosts of that server.