error WIX0150: Undefined preprocessor variable after upgrading to WiX v5

[hypetsch]

We recently upgraded our WiX v3-based installer to WiX v5. The .wixproj has multiple <ProjectReference> elements to other .csproj projects to access their preprocessor variables like $(Project.TargetDir). All of these variables now cause a WIX0150 error.

The .csproj projects are built upfront using a different solution and are just referenced in the .wixproj (and the "installer" solution) to have access to those variables; "building" the referenced projects is disabled in the "installer" solution file.
In WiX3, this worked fine, and the variables were defined anyways, whereas in WiX5, they seem to not get defined when the referenced project is not built as part of the solution.
When building the .csproj is enabled in the "installer" solution, the errors disappear. Sadly, doing this is not possible for us.

We're not sure if this change in behavior is intended (which we hope is not the case) or a bug in v5 (or maybe also v4)?