I'll try to explain the meaning of my changes for maintainers to help improve it.
The whole problem is that, for example, MultiDict ({'x': 42}) inside dict.items is interpreted as {'x': [42]} !!! Especially for this case, the flattening of arrays of one element was implemented in the code. The problem was that it does not take into account the fact that the transferred object can be an ordinary dictionary, as a result of which the bug described in the issue appeared. In 1.0 version, this bug was not present, since work with MultiDict and a regular dictionary was done separately. I added to the code extra check for MultiDict to avoid strange behavior.
Any feedbacks?

This was on the right track, but I had some more I wanted to do with it, and it needs tests as well. Continued in #2307

Hello, @davidism. I'm glad I was able to fix the bug myself correctly. Thank you for clarifying that I think correctly ("on the right track"), this means that I really began to understand something about WerkZeug !!! It is a pity that I myself cannot continue what you described (I wanted to do it tomorrow) ...

Your commit is in the other PR, I just was doing work today.

Cool! I see the commit. Understand you)