Collection issues: mysteries, objects vanishing from one and appearing in another/ kitbash3d

I’m experience baffling behavior while simply trying to neaten up a scene using Collections.

What it is is a KitBash3d set, which loads as a 3x6 array of assemblies that you can use to do whatever you want. Each assembly may be comprised of several objects. So, in Object mode I’d box select with Xray on to get every Object in a given assembly, and then over the Outliner hit “m” and add it to a New Collection, labled “Rncn” for row and column.

But quite soon in this process, Objects started disappearing out of their Collections and being added to the new ones, even after I triple checked to make sure nothing was Selected before creating a new collection.

Not sure what’s up, except maybe hidden selection in the Outliner (it’s quite ‘tall’) at this point. I’ve been unable to construct a blend file that would replicate this bug, as it would be quite laborious to replicate the same situation (without compromising KB3D’s ip).

Anybody experience anything like this? I have a sneaking suspicion it’s pilot error but I’m not sure where/when.