It seems the detection pin for M.2 SDIO interface is missing. Currently in order for M.2 SDIO to be read/written, a dummy micro-sd card should be plugged into the micro-sd slot on the board. Basically the M.2 SDIO detection replies on the DET PIN from micro-sd card.
Is there a proper way for M.2 SDIO to work without micro-sd card getting involved?
See the schematic attached.