As pointed by Don N2VGU in #82726, NanoVNA is not able to fully drive the amp so the impedance measurement is inaccurate. I just did that and I know for sure driving it with 705 or KX2 @ 4W would have SWR of around 1.8, but measuring it with NanoVNA gives me 23.7ohm/1.09nF, SWR=2.2 which is pretty close to your result. But my PA can deliver 40+W with about 5W input power. So your problem is probably not related to the low input impedance.