See this Forum thread for the details:
https://forums.adafruit.com/viewtopic.php?f=57&t=182604&p=887344#p887344
Summary:
with an RFM9x featherwing mounted on a nRF52840, all works normally when powered vis USB, but when powered via a LiPo battery, the rfm9x fails its version check.

The OP found a workaround:
create a dummy SPI instance before creating the real SPI instance that is fed to the rfm9x init.
This appears to force the nRF52840 to use a different SPIM and it works. It is not clear why....

This problem is not present on M4 or esp32S2 boards.