I found one other post that seemed to address this issue. My problem doesn't seem to be any of these things.
"As for the engineers not repairing - had the ship you wanted them to repair been damaged within the last 3 seconds?"
nope.
"Do you have sufficient resources to enact repairs? Does the unit state 'Can't be repaired' in its tooltip? Some simply can't, such as fortresses."
nope, I can repair if I manually click on the unit.
"If you want them to repair a specific ship, I recommend ordering them to repair that ship explicitly. If placed in attack-move mode, they may teleport away to focus on repairing another ship first."
That's fine, but they aren't repairing any ships, or helping in construction, or doing anything.
I'm pretty sure I'm issuing the order correctly. I've tried with and without auto-FRD, hold V and right-click within repair range, hold X and right-click within repair range. I don't know what's going on.
I've attached a save. Thanks in advance!