To build on GoldingD's reply, that stacking behavior is by-design:
https://helpx.adobe.com/lightroom-classic/help/grouping-photos-stacks.html
"Stacks are specific to the folder or collection in which they were created. The photos in a stack must all be from the same folder or the same collection; you cannot create stacks while in a smart collection or a published collection.
"You can only view a stack in a folder when that folder (or other folders) is selected as a source, or when All Photographs is selected. You can only view a stack in a collection when just that one collection is selected as a source. Stacks are not visible when a mix of folders and collections is selected."
See here for precisely how weird, complicated, and confusing LR's stacking rules are:
https://community.adobe.com/t5/lightroom-classic-discussions/lr-classic-v8-2-1-stacking-missing-from-context-menu/m-p/10459188#M146006
See here for the 2011 feature request to make stacks global, not tied to particular folders or collections:
https://community.adobe.com/t5/lightroom-classic-ideas/lightroom-stacking-in-folders-and-collections-should-be-global/idi-p/12250295
That's going nowhere -- I don't think Adobe will ever change stacking behavior at this point, given how little they invest in Library.