This seems to correlate with my upgrading to the latest Win10 SDK on those workers (Steve pointed out I was still getting ucrt warnings during compilation).  So they both jumped from like 10240 up to 16299.  But that's all I changed.

Interestingly I only had to fix 8.1 and 10, as 7 already had a version (15063) avoiding the warning, and it's tedious to change.  But it's not experiencing the linking problem.

I could swear I recall past link/rc problems, but my searching (and memory) are failing me at the moment.  But I wonder if that's why I had left these two workers back at 10240, or if I had some local fix in place.