Mason jars are microwave safe although you need to be careful about temperature changes or the mason jar could break.
As long as the mason jar is at room temperature and doesn't go direct from cold to the microwave it should be fine in the microwave.
Newer mason jars are more microwave safe than older mason jars.
Newer made mason jars have a symbol on them that indicates they are safe in the microwave.
Just be sure to remove the metal lid as the metal lid is not microwave safe.
Also don't microwave the mason jar too long as the glass reacts to the heat and could shatter.
When microwaving mason jars they should not be microwaved longer than 5 minutes.
And do not microwave frozen mason jars as they will shatter.
Mason jars are made from tempered glass and can withstand quick heating.
I heat things up in mason jars a lot and haven't had them break.