I found it surprising because I interacted with their REST APIs first. They are very good about docs. The ones I use most frequently (TS) are no exception, though they have some gaps. But the actual REST APIs I integrated very clearly predate a lot of their guidelines or struggle to follow them.
A Microsoft are very good at telling you (usually) what's there but they don't like telling you what's missing or what's inconvenient to use. Or indeed, what they have downright deliberately broken.
The documentation quality for MSSQL was very good in 2000, and noticeably has deteriorated since then to the point that I (who carefully read the docs) actually released some broken code related to the MSSQL geographical data types (edit: this was in MSSQ 2012). Fortunately cleaning that mess up in the morning while clients were phoning us wasn't too bad, but it could have been.
I also notice that they've made the offline help both hard to get to (you have to install it manually now as a separate process) and inconvenient to use in various small ways. I am certain this is an attempt to push you to use the online help instead. As a professional dev I find this objectionable.
Example: learning C# and their examples didn't ework. Someone else on HN replied and said the same. Turns out they had moved the LINQ functional stuff into a separate module you had to import. Once imported, everything was fine but their examples were broken and there was no way to know what to do - a very simple solution was to get the compiler to recognise that certain stuff was missing when you try to use it and just tell the user. But no, they left you to flounder instead.
Another example: I lost ~1 day trying to start a C# GUI project that just would not show data binding components. Turns out that it had to be a particular project type (not .NET core IIRC) but that was not made clear.