15uA would be about right for a Ublox in software shutdown, but I was going on the GITHUB for the RAK 5206 which says they are using a L76. This matches the schematic.
If they swapped to a Ublox MAX 7Q, then they ought not to have left Vbckp floating, from the 7Q datasheet;
“If no backup supply voltage is available, connect the V_BCKP pin to VCC_IO (or to VCC if not avaiable)”