Further comments and a workaround for posterity:
I fought with this this morning and came to the same conclusion that “InteractableWhenSelected” and even “Toggleable” are not intuitively named or designed. I too have a set of four buttons and exactly one must be selected at all times. Toggleable must be true if you want to be able to deselect other buttons in blueprint. InteractableWhenSelected set to False however does not make the button non-interactable (and what it actually does is a mystery I haven’t dug into). Other solutions, like trying to shut off Toggleable when selected or making it non-hit testable when selected also didn’t work for unknown reasons.
Instead, I added a variable that tracked what button is selected (which I use anyway since in my case these are category buttons that filter a tile view). In On Selection Change, I check that selected category and if Selection Change is trying to deselect the button while the category is still the selected category then I reset it to selected. I abstracted this handling into a function so all of my buttons could call it, reselect themselves if needed, else deselect any other buttons (in my ResetCatSelection function), refilter the tile view, and save the updated category.
Making InteractableWhenSelected actually work on Toggleable buttons would be ideal but this now operates as expected.
