In case anyone else runs into this issue, those tabs are only for the tooth on the XLR cable to lock into. It's not a major issue if the tab is missing, you just won't have normal XLR locking action. I ended up fixing an issue with my tabs, but first a warning:
DO NOT TRY THIS AT HOME, ESPECIALLY IF YOUR HELIX IS UNDER WARRANTY.
When I got home from rehearsal Sunday night and pulled my Helix out of its case, I heard a tiny loose part rattling around inside the Helix itself. So I pulled the old pick-inside-the-acoustic move, holding the Helix upside down and shaking the loose part towards the opening for the expression pedal. Out popped this:
After scratching my head for a second, I realized it was one of the tabs at the top of the XLR outputs. Sure enough, one was missing and one was pushed halfway in:
Compare to my gig backup Helix:
I submitted a support ticket with L6, but since my Helix is way past warranty and would cost me shipping and bench fees, I decided to fix it myself. Opening up the Helix isn't too hard if you can work a screwdriver and Allen key, but again, DO NOT DO IT IF YOUR UNIT IS UNDER WARRANTY. I don't know for sure about the Helix specifically, but with electronics generally you run the risk of zapping something with static or breaking a connection if you're not careful. Anyway, opening up my unit, this is what the top of the XLR connectors looked like. You can see where one tab is coming out and one is gone:
The tabs are just a simple press fit, nothing clicks into place, so it's not too surprising that after 4.5 years and lots of rehearsals and gigs with random house XLR cables they would start to work loose. Pushing them back in was easy:
I also ran some CA glue between the tabs and the connectors to try to delay any repeat of this happening. Incidentally, on this unit one connector is an Amphenol and the other is a JT. The Amphenol has a little spring-loaded wheel: that's the switch that tells the Helix the right XLR is being used. You can just see it on the inside of the left wall of the right connector:
Back to normal: