I’ve typed this answer twice and it keeps disappearing. I’m going to try again!
The problem with a microwave kiln is that you have so little control over your firing. I have no idea what a “medium” or “high” temp firing is in a microwave kiln but since it does appear that you might be overfiring why not try lowering you temp or shortening your time until you find what you want. Seems like the thing to do!