Chris, there was a parent.
Gilman-12 had a parent Gilman-29
Instead of adding Gilman-163 as a child of Gilman-29, I clicked on "add a sibling" on the edit page of Gilman-163 (the reason I'd done this is because all of the literature I'd been reading had led to the conclusion that Gilman-163 was a *sister* of Gilman-12, so I was in a sibling frame of mind).
After the sibling attachment was completed, Gilman-29 (as father of Gilman-12) was replaced with what I presume to be a brand-new profile, Gilman-598. Gilman-598 also showed up as parent to Gilman-163.
BUT, oh i think i'm getting it now. Gilman-163 did NOT have a parent at the time I added her as a sibling to Gilman-12 (who did have a parent). So wikitree saw "no parent for Gilman-163; we better create one" instead of seeing the existing Gilman-29 as a potential father. I can understand the creation of a new profile for a fatherless sibling, but I don't see why it replace Gilman-29, an existing parent.
In any case, I place this topic here as a warning to NOT use ADD SIBLING, even though the command is there, and the subsequent page allows you to attach an existing profile.
What would be great is if when you try to attach a parentless child as a sibling to anyone, it stopped you from doing so. Ideally, it would recognize, oh, [target sibling] has a parent; are these the appropriate parents for [attaching sibling]?
And if not that, then just prevent Attach Sibling from parent-less, existing profiles.