This is almost certainly because the popover button is OVER rather than IN the portal. This can usually be fixed by nudging the button a little up, down or sideways—guides will probably appear to indicate if it is propoerly captured by the portal. You can test this by moving the portal itself—if the button moves with it, it is captured; if it doesn't nudge it until it is. Repeat until successful.
My suspicion as well, keywords. Generally, the new object containment model introduced in 12 has been a huge positive, but this phenomenon is one niggling issue with the model. I wonder if FileMaker shouln't consider introducing a "layers" palette, ala most image and vector graphics apps out there, for management of containment, grouping, and stacking order, as well as easier selection of layered objects. Perhaps time for a trip over to FM's feature suggestion page…
Also, Draco, you may or may not have seen this excellent seedcode blog on popovers, their limitaions, and quirks. If not, it's worth a read as you play around with popovers.
thank you, Mark and keywords