When they stop, do the Things show as online still?
Ultimately I think this is going to need an issue and a maintainer to solve given there’s nothing in the logs that help.
As a temporary work around you can use something like Thing Status Reporting [4.0.0.0;4.9.9.9] (if the Thing goes offline) or Threshold Alert and Open Reminder [4.0.0.0;4.9.9.9] to detect when a Things stops reporting and reset it from a rule. But that’s just to get you by, not since the problem.