I could not create any MS with neither Brit universal carriers, near the SW Bruxelles bridge with a DO on it. It kept saying at any distance that I was too far from mission target, which I was not. I tried again after restart, with two different accounts and from two different bridgades/fbs.
actually it also doesnt work on other DO bridges/towns/vehicles, so its seems related to DO on bridges not being recognized as target.

Mobile spawn targets must a “owned” target. Must be a town or depot. They work link to link so must be targeted to a linked flag.

