I ordered a collier ship to visit a nearby system, dump ammo, return, and rejoin TG "1st Fleet" at Sol. The ship's name was "Tender 01" and the hull abbreviation was TN. The ship was alone in a TG called "TN Tender 01".
While the collier was off doing his thing, I ordered TG "1st Fleet" to join TG "Earth Shipyard". These two TGs were at the same location, and this order was completed before the collier returned.
When I queued the 5-day pulse on which the collier was to reach Earth, I got a couple error dialogs that I just clicked through, hoping that the game would keep going. In the Event Updates page, I got an "Orders Completed" message stating "TN Tender 01 has completed orders".
I think TN Tender 01 vanished. It's not listed as a member of TG "Earth Shipyard" and it's not listed on the Individual Unit Details pane, nor is it listed on the "Ships In Class" tab for its class on the Ship Design pane.