It seems that recently we have added an unconditional upgrade script causing all existing sites to change their default behavior from defaulthomepage = HOMEPAGE_SITE to HOMEPAGE_MY. See (part of 5472f9f4e, belonging to MDL-45774):

https://github.com/moodle/moodle/blob/master/lib/db/upgrade.php#L4340

And that's incorrect. Existing sites should not get their previous behavior changed. NP with new sites if documented. That was achieved (correctly) by MDL-49630. See:

https://github.com/moodle/moodle/commit/9e2b644

So, while the later is 100% the expected behavior (apply new default for new sites, keeping existing ones unmodified)... the upgrade step above destroyed it causing upgraded sites to change their behavior.

Not sure if that was discussed at some point, but IMO it's simply incorrect/opposed to our usual BC rules, leading to confusion, hence reporting it.

Ciao