Nondeterministic list archiving behavior

[clonm]

clohp identified an issue with archive lists for fall 2019 and spring 2020:

An alum asked me to remove her from announce@cloyne.org and add her to the appropriate alumni lists. In doing so I saw that the full alumni listserv, as well as the lists for Fall 2019 and Spring 2020, are incomplete. She was also on the current announcement listserv (but not clones@cloyne.org), in case that's useful info. Could you please let me know when you've had a chance to review and fix this?

@bhoberman did some testing with dummy lists and found nondeterministic behavior.

This may have been fixed in one of the various improvements to data source behavior mentioned (in these release notes)[https://github.com/sympa-community/sympa/blob/6.2.56/NEWS.md]. In any case, we should upgrade first before continuing to debug.