We have a presence sensor on the dog. Sometimes, in the middle of the night, he vanishes for 1-15 minutes and then suddenly reappears. The only wall-powered zigbee device we have is the hub and he usually sleeps in the bedroom above the hub so he’s not far away. I’ve been speculating that he’s in a position where his body blocks the signal. Directly above the hub might be a null in it’s antenna also, depending on the type of antenna it uses.
By comparison, the smartsense sensor on the mailbox is much farther away and has been completely reliable.
None of that explains your behavior but we do see similar symptoms.