Ok, so after working with Mike M. Volokhov (@mishkathebear on Twitter), it seems design 1.0.4.ty.01005 is a no go for Wi-Fi. If I understood correctly, it resonates at too high a frequency (sorry, I'm still learning). Here's the Smith chart and SWR, MAG plots with the current chip antenna and footprint.

...I will have to make another FEMU revision without the cutout, and attempt to make a more suitable ground plane clearance, perhaps with a different chip antenna.