This happened to me in a web form, but it was with .jpg and .jpeg. I was on a phone.

In what capacity?

The built-in APIs for handling images (GDI+) added WebP support in 1809. This is mentioned in the the documentation for the drawing library in C#/.NET, which is a wrapper around GDI+: https://learn.microsoft.com/en-us/dotnet/api/system.drawing.imaging.imageformat.webp?view=net-8.0

I haven't tried it in the Photos app recently, and don't have a Windows system handy right now to try it out, but I know it works for sure in Paint, which uses GDI+ for image encoding/decoding (and thus supports all formats that Windows itself supports).

Of course, apps that use their own encoding/decoding code might have fewer features and support fewer formats than the native Windows code.

Edit: Apparently it works fine in the Photos app if you just change the extension? Which means Microsoft is doing something weird. The code to decode WebP must be there.

PNG offers sharper images, which is more important than compression when you have complex diagrams

WebP supports both lossy and lossless compression. Diagrams should use lossless compression so the image does not lose any quality.

PNG gives better performance when rendering

Images on the web usually aren't large enough for this to make a significant difference, and it can sometimes be offset by the quicker download time.

because of CVE-2023-4863.

libjpeg and libpng have had a number of CVEs too though.