See microsoft/WinUI-Gallery#1349
Niels was able to test this in a file-new WASDK project template with the WAPProject and see the same issue.
Something in a wapproj is pulling in the UWP references alongside the WASDK ones.
We'll have to follow-up with the platform and get/track an issue for WAPProject in general, but for now the easiest thing is for us to go back to having the Uwp/WinUI versions of the packages not include the other target.
I think we'll want the Uwp one to not include the WASDK as well, so that'll be new build steps... Will see how tricky that is.