Okay, it just happened again, although a little different than before. I don’t have repro steps but do have a screencap to show it. Will keep looking for repro. Note this example does involve mirrors… The mirrors shown in the example are – I’m pretty sure – “originals”.
Screencap shows first doing multi-select from the first child to the last, using mouse
then it shows same thing using shift+downarrow
In both cases the parent AFTER the children gets auto-selected instead of the children’s actual Parent.
Also, not shown: If you initiate the drag on the parent and drag down to the first child, multi-select automatically expands (as it should) but extends all the way to include the next parent, instead of just up to the last child.