From my experience (on Chateau LTE12) when lte antenna is set manually, in Winbox “External Antenna Selected” is empty (this field is only for auto selection as mentioned here for external-antenna-selected), but from CLI:
/interface/lte/settings> print
mode: auto
firmware-path: firmware
external-antenna: both
When is set to auto, ROS decide which to use (internal or external) depending from which antenna has better signal (RSRP) as mentioned here for external-antenna.
But in my case when auto is set it always select only main not both, even I have two connected HGO-LTE-W antennas, one on main and other on div and I must set manually to both.