It is not unusual for microwave kilns to change color so I wouldn’t worry about that too much.
What I think happened is that the glass heated too quickly and the top piece moved off and broke. When you fuse two pieces together you must increase the heat very gradually until they are fully fused. This is not easy to control in a microwave kiln and even though your other pieces were successful each new piece can vary. Whether a piece has been fused before makes no difference.
The big problem with microwave kilns (I have one, too) is that you cannot control the temperature which is essential for successful fusing. I use mine to test glass color or to make glass dots, but my serious fusing is done in larger kilns.
If you are serious about making fused glass jewelry, you might want to consider a small table top kiln which costs under $500.