Was just playing execution before on Overground.
Started on the attacking team. Armed the C4 at site A. The C4 disappeared and both the pylons closed (like the ones usually do that you do not arm at) and that was it. Nothing could be done from there.
The other team killed us all so you would think the round would end, but the round kept going as the game must have thought the C4 was still armed, but when the round timer ended it gave us the loss any way.