(I've only tested this on SP)
When you put frozen food into a microwave (set to defrost), it takes about 15 minutes to defrost while you are actively looking inside the container. If you walk away from the microwave/close the loot window it doesn't defrost the things at all, the freezing bar progresses as much as if you just left it in an unpowered microwave/any other container.
A quick play showed that its the same on all of the heat settings of the microwave. I have no idea why this is happening, stoves defrost things fine from the quick mess around I had with them. I haven't played around with actually cooking things with microwaves though, maybe I need to look into that as well.
I first noticed this on a modded run. I disabled the mods to test and it still worked with those microwaves. I then disabled mods and started a vanilla new world and tested with fresh microwaves there and it was the same. I have no idea if there is something permanent mods have done, but I got it to replicate on a fresh world in SP.
