Oh, I had incorrectly skim read @OptimisticMonkey’s solution. When I read “Maximum number of parallel project builds” I stopped reading and assumed it was going to be setting MaxProcessorCount to 1 in BuildConfiguration.xml, which if was true would have pointed to a UE4 build bug. But this parallel build option in VS is a global setting, not per project. I didn’t even know such an option existed.
Although I can understand why this would mess up Unreal Build Tool, it would be nice if you got a warning about this (perhaps during VS project file generation), or if Epic said to set the value to 1 in the UE4 Visual Studio Setup Guide or github README. I’ll try this solution out and if it works for me, I’ll make a pull request on the README.