This is what I see in Windows File Explorer - but only with Icaros activated - for your webm file:
Without Icaros, there are no properties visible at all.
You would have to ask Microsoft about the reasons.
I would say that this is not a Mp3tag bug.
It's the usual (very limited) behaviour of Windows File Explorer.
Mp3tag adds the tags to the .webm file, save them and let us change it as expected.