Not sure if it is a bug, but I would send the coastal helicopters out to missions, it would arrive and then leave. And the mission would never get completed. I tried assuming that the program might consider them ambulances in one way, so I had to change the dispatch center setting to uncheck "Automatically stand down EMS units when not needed." and that took care of the problem.
I use rely on that feature to kick loose unneeded ambulances so I thought I would like to see if the Coastal Helicopters can somehow be excluded from the EMS Stand down option.