I launched a sub of 40 drillersagainst an enemy outpost, and cameback an hour later to find it had either made little or no progress to the target... When I select it, it doesn't say when it will arrive or give the option to cancel, it just says "Combat in progress" and has a countdown timer. The timer starts at ~8:30 minutes, but whenever it hits zero, it restarts.

And as far as I know, there are no subs launching between the two outposts in question besides mine, although the enemy outpost launched a normal sub at a different outpost in the other direction.

I can see a sub doing this as well. But looping on 10 minute timer for the Combat in Progress... 10:00

Has a Navigator on board. Does not seem to be moving, but only 21 minutes out.