Exception VEVENT blocks should not inherit unchanged custom properties from the master VEVENT block. For example, if a recurring event has had a Thunderbird / Lightning reminder dismissed, it gains an X-MOZ-LASTACK property in the master VEVENT block.

If you then modify a particular instance of the event (e.g. your attendance status), producing an exception VEVENT block, it inherits the X-MOZ-LASTACK property. This doesn't really make sense - that instance has not had a reminder dismissed - and also causes issues in Thunderbird / Lightning.

Unless the custom property is modified as part of the exception, there should be any need to copy it to the exception VEVENT block.