Vampyr

Sorry... I stand corrected. LOS was not the issue. It's simpler than that. I simply wasn't following the JTAC's instructions correctly, and that was resulting in them stopping the lase upon weapons release. I would actually hear them say "abort" but couldn't figure out why. For some reason I thought that they'd continue lasing after aborting, which is not the case.
Chalk it down to me not understanding the JTAC's instructions.
If the JTAC loses LOS and stops the lase, they'll mention something along the lines of "no mark". That's very different from what I encountered.

Problem solved.
Contrary to what I read on a different thread while Googling for solutions, it is necessary to contact the MQ-9 over radio to get it to lase the target, same as any other unit carrying out JTAC. I was under the impression that radio contact wasn't necessary with the MQ-9. Kind of makes sense I guess, given that the MQ-9 can be controlled by a human pilot in the real world.
I also revised my WP tasks to just the following:
EPLRS on Weapons hold FAC assign group I removed the orbit task because the MQ-9 will automatically begin orbiting the target when given a FAC task.