Wow, PROCESS_MODE_BACKGROUND_BEGIN is quite the footgun. I wonder if this started out as a hack to reduce the impact of OEM bloatware on benchmarks. Is there an easy way to list all processes that use this priority class?
There probably is a way to find what processes have this mode enabled, but it's probably more productive to search github for references to the problematic constant.
Run Process Explorer and enable the Memory Priority column. In the Select Columns dialog, its checkbox is on the Process Memory tab. Priority 5 is normal; priority 1 is background.