Attempting to double-patch a MC fixture in (2) separate universes and think I've discovered a bug -- The bug is not appearing with non-MC fixtures.
Channel 511 / Part 1 (in this case, a Chauvet MK3 Wash in Single Standard MC ) is patched at [UniverseA] / 1-129. Its' respective .cells appear as expected in patch.
Channel 511 / Part 2 is created (with the default type of Dimmer). All is good here.
Channel 511 / Part 2 type is changed to [any fixture type]. 511 / Part 1's .cells all disappear from patch.
Yes, indeed. It looks like someone else mentioned it a couple months ago, too : b82 Adding a Part 2 to a multi-cell fixture deletes the cells - Eos 3.1.1 Open Beta Bug Report - Eos 3.1.1 Open Beta - ETC - Community
This is written up as [EOS-53199] Multicell - Patching Second Part with a Fixture Type removes all cells from Part 1.
There is a workaround: Either always patch the part 2 first, or move the part from another channel onto the one you want.
Well, that's embarrassing -- I apparently did not do a great job searching around before posting. Sorry about that.
(And thanks for the work around)