Yep - I assumed it was Tesla mothership to the car rather than teslamate to anything directly.The offline state comes from the Tesla API rather than Teslamate. Teslamate just reports the state.
Teslafi advise that offline means the car has lost connectivity to Tesla's servers although that may be an assumption on their part.
Perhaps there's a disconnection process when a car sleeps e.g. it tells the Tesla servers it's going to sleep. If it just loses the connection it may not have had that disconnection message and marks it as offline. As you say there are plenty of areas that have poor reception, so marking the car as asleep every time you lose connectivity may not be the best course of action.
Anything in the logs that look suspicious? Might be worth comparing a normal sleep with the offline event.
IIRC the last time I had this I went into the app and then came out again and it changed to sleep. (maybe online for a bit and then sleep - it was a while ago)
I’ve found a couple of other occasions and your idea of it maybe being linked to the mothership not receiving a “off to sleep” message could fit. None of the offflines are in the middle of a recognised sleep.