Hello Victor.  Thank you for looking into this.  I ran the attached script, sigusr1.py as you requested.  The output was as expected:

[jshelly@gl-build bin]$ ./python3.7 ~/sigusr1.py
called True

Not sure if that helps identify the problem.  Regarding the related bugs I mentioned, I think my first comments were mixed up.  There were 2 possibly related bugs.

1.  Redhat bugzilla 1687171, which may have been related to the skylake chipset: https://bugzilla.redhat.com/show_bug.cgi?id=1687171 

2.  Github # 32:  https://github.com/vstinner/faulthandler/issues/32
That is an older one (2017), and maybe not related.

Bug # 36 at Github is one that I opened initially, and that one is a duplicate of this bug report. It is closed now as I was asked to open the bug here instead.

Thanks for any further insight or suggestions you can provide.