I spoke with Alarm.com again and they had to do some digging but a higher tier tech stated that this is actually a rare issue they have encountered before with the Wifi hub in particular, where ADC will send commands to the hub, but only one works (presumably just the first one to be received). This lends to the random nature and makes it harder to identify.
It is an issue between Liftmaster and ADC. They are working with Liftmaster to fix but there is no ETA.
There may be a couple work-arounds if you want control through Alarm.com and simultaneous opening/closing:
- Linear Z-wave opener may be an option depending on your system and panel distance.
- Do your liftmaster openers have a MyQ logo on them? I am curious if they might be compatible with the 828 LM. They might not be based on the manual, but worth a look.