The pointer by Pete toward a more global approach was helpful and led me to run a
'fixfiles check'. This revealed lots of contexts needing restoration,
particularly in my home directory. Seems to suggest that running fixfiles is a good
preventive measure to perform regularly.
It was also suggested to me offline that having restored the proper context for
.Xauthority, it was no longer necessary to have a particular local policy in effect for
my-sddmhelper. So it could be deleted with 'semodule -r my-sddmhelper'.
The origin of the problem seemed to arise from running package updates, but exactly how
will remain a mystery.
Learned more about the tools and thought processes for understanding SELinux problems.
Thanks all.
-- Bob