Xcode cancels running build for no apparent reason?

I've noticed some weird behavior in Xcode and am trying to diagnose it. I start a build and after compiling a file or two, the build is cancelled. For no apparent reason. No errors etc. are displayed. Restarting the build (once or twice) will eventually get it back to running through as normal. I've seen this issue for the last couple of Xcode versions off and on, and it's an annoyance, but in the most recent Xcode 12.1.1 release I've gotten to a state where no amount of restarting the build will get past it. I've tried rebooting OSX, and leaving my machine for a time to cool off (I was wondering if my CPU was overheating). No change though. No other software on my machine is misbehaving. Just Xcode.

Has anyone else seen this issue? Any ideas for how to diagnose this?

Some details of my setup:

• A large project with mixed ObjC/Swift/C++. A number of CocoaPods and C++ libraries. Approx. 30K source files overall.
• A mix of older Xibs and newer Storyboards (unsure if it's related or not to Xib processing)
• I've tried using both the new and old build systems
• I've tried cleaning out the DerivedData on this machine
• I don't see anything over

Thanks for reading. Any suggestions for things to try here would be much appreciated.