My expectation is described in the comment you replied to and side comments

Microwave's ding is also bad UI, especially when it's repetitive as it's just useless noise. But even the first ding - what value does it have if you already have exactly the same notification category - audio cue - built right there in the process: the machine noise disappears when it stops working.

The microwave in my house continues making the operating noise after it finishes microwaving.

So? Different models can require different UI?

Probably the noise of the cooking fan running a bit longer to cool down the electronics again.

cooling* fan

Waiting for noise to stop is less reliable because of how brains work, and also has a much smaller range. The microwave ding is useful.

Brains work just fine and your typical kitchen/apartment range is small enough, but in general - the trade-off between comfort and reliability is not something you can resolve for others

When the goal is "notice a background noise going away while you're involved in a different task" brains do not do just fine.