I've been using a module with a passive on-board antenna and an SMA socket.
From here:
link to supplier's page
photo:
The supplier advises:
"Please note: When connecting an external antenna the C2 capacitor on the module needs to be removed. If the internal and external antennas are used together they will interfere with each other, affecting positioning."
Does that have any bearing at all on the situation with your board?