The ProtocolInfoModule was created to address a limitation in IIS where the SERVER_PROTOCOL and HTTP_VERSION server variables do not accurately reflect whether a request is using HTTP/2 or HTTP/3.
I suspect someone at MS forgot about this (somehow) when completing the work for HTTP/2.
To use the ProtocolInfoModule, download the release, drop it in your %windir%\System32\inetsrv folder, and install using IIS dialogs.
If using kernel caching you may find HTTP_X_SERVER_PROTOCOL is only correct for the first few requests, there is no way around this short of disabling kernel caching, sadly... MS will just have to fix this eventually (I hope)