It appears there are a few topics that publish information about a thermostat. One appears to possibly not include information about schedules, so the plugin defaults to an empty list.
I think the default should be changed to None, and when a None value is read (the TRV should ways produce a list), the internal list should be left unchanged. If the TRV has its schedules cleared, then it would report an empty list which would then be replace in indigo.
It appears there are a few topics that publish information about a thermostat. One appears to possibly not include information about schedules, so the plugin defaults to an empty list.
https://github.com/AaronLionsheep/ShellyMQTT/blob/d0c6659acc40f33cd2e629f88bdb7a9ec0d28123/ShellyMQTT.indigoPlugin/Contents/Server%20Plugin/Devices/Shelly_TRV.py#L120
I think the default should be changed to
None, and when a None value is read (the TRV should ways produce a list), the internal list should be left unchanged. If the TRV has its schedules cleared, then it would report an empty list which would then be replace in indigo.