Alright so i just had this bug happen to me.
This is how it went down.
1) engaged Pirate Plunder Fleet.
2) Won, however several pirates fled the field
3) Didnt bother to continue pressing, so moved to Salvage.
4) The pirates attacked again. (in dialogue. never got kicked to overworld in this whole encounter)
5) defeated one ship
6) combat complete
7) move to salvage
The pirate fleet attacks!
9) Pirates field no ships
10) Combat complete!
11) Move to salvage
12) The Pirate Fleet attacks!
13) pirate fleet fields no ships
14) combat complete!
15) Move to salvage.
Ad nauseam
If you choose to harry as opposed to salvage, the same battle opens. opposing fleet deploys no ships, combat over.
If you choose to stand down. you get no loot from any of the fights.