I have a US915 Base RAK19007 with a Core RAK4631. It works with 1.14.1 MeshCore as expected. However, I also have a RAK12014 Time of Flight sensor (measures distance). When connected to Slot A, it works perfectly (although pointed up, which will not work for water). However, when connected to Slot C (which is on the underside of the board), the distance values as reported by Telemetry is all over the place. It will show 17.753m, then the next poll it will show 7.147m, etc..
Per the datasheet, this sensor should work in Slot C.