It is an interesting question, and Lady Waynwood is not the only case like this – there’s Barbrey Dustin (albeit without an heir), Arwyn Oakheart (whose youngest son was Ser Arys), the Hornwood Crisis, Delonne Allyrion (who has an adult, and indeed married, son), Larra Blackmont (who has two I think adult children), and so on and so forth.

Trying to reconcile this with what we know of Westerosi succession laws is tricky. In some cases, I would guess that a lot of those situations are ones in which an heiress of a house continues to rule until her death (same as a male lord would do), whereas the widow of the previous lord would normally hand over the title to the lord’s heir (although, as we see with Lady Hornwood and Barbrey Dustin, there are widow’s use–rights in cases where there isn’t a clear heir.