It looks like it comes down to 4x4 MIMO kicking in and using both external and internal antennas which triggers dropping of CA bands because of the low signal quality for internal antennas. I don’t think 4x4 MIMO can be disabled with AT commands. This seems to be available only for EM160 with AT+QCFG=“lte4x4mimo/disable”, but there is no such command for EM12/EG12/EG18
any assistance from mikrotik side would be welcome