Eu não tive esse problema, felizmente, mas eu acho que se parece funcionar ok, então você pode continuar usando-o. Depois de um pouco de pesquisa eu também encontrei this :
I accidentally put something metal in my friend's microwave about 3
years ago. The metal was covered in plastic so I didn't realize what I
was doing. The smell was really bad, but to this day, the microwave
works.
My mom's microwave was having a problem where it would not turn off.
The other day, she was on the phone and forgot about it, and burned
something up in the microwave. Now it won't even heat water. So, she
had to throw her's out.
I guess the microwave is okay if it still works.
E semelhante aqui :
The metal isn't going to hurt the food in any way, the food should be
fine. As long as the microwave seems to function properly, it should
be ok, as well. I have accidentally left utensils in the microwave
before and the appliance didn't sustain damage. Damage is just a
possibility, not a definite occurrence.
Some metals are even meant to be placed in the microwave, it depends
on the type and the shape. My microwave, for instance, came with a
metal rack that slides into the center. I think it depends partially
on whether or not there are flat edges on the metal, which can cause
sparks.
Bottom line, you're ok. Just don't make leaving metal in the microwave
a habit. Technically it could short out or catch fire or otherwise
start acting up. But you'll know if that happens.
Acho que outra boa ideia seria ligar para o fabricante e perguntar se você quer verificar novamente.