What may be missing (and I wasn't clear about, perhaps) is that many of these "proposed matches" have been hanging around for quite a while.
I can't say why that merge ended up on the list, nor its "history". I can't say whether or not Jason or someone else proposed that merge -- or possibly a merge explicitly made somewhere else on Jason's branch. But being on "the list" means Geni has authorized the curators to validate the match -- and complete it if the match seems correct.
Lois: In this case the issue of a 'detached' branch is not in question because it seems (as best as I can tell from Revision history on some of the profiles) that Jason's branch that was originally created 'detached' (not part of the World Family Tree); that is, Priscilla & Dillon's family profiles were already a part of the World Family Tree.
Regarding 'returning to be a separate tree': Again, not apparently relevant to this situation ... but, as a point of note, while curators can 'undo' a specific merge (if it was done less than ~6 months ago), that does not 'undo' the 'connection' into the World Family Tree (in other words: the "big tree" flag, once set, can't be un-set by curators; only by Customer Service).