Wow this is weird. We didn't do anything on our end to do that. I just checked both my machines and both had affinity set properly (all CPUs enabled).
I'll keep asking around the team to see if anyone else has this issue.
For the record, OP is right. You should have all CPUs enabled in the affinity mapping. If you have this problem, maybe try listing your CPU + windows version to see if there is a commonality? You can get your windows version by pressing WinKey+R and running "winver". ex: Version 1803 (17134.1488)
P.S. There is a bug megathread stickied on reddit for each patch. It wasn't a beta only thing.