F5 doesn't help, and there's no way to cancel the troop movement, which is not a bug but annoys a lot, especially when playing using a smartphone with a high probability of a misclick on a tiny battlefield with curvy borders - we'd propose to implement a possibility of undo for any action which doesn't require a token draw or reveal. Or at least a confirmation button for any action, as an option!

This is the current English text:

Clean Sheet
Choose a Territory: remove all Fighters present there (yours and your opponent's).
These Fighters are returned to their colored bags and can rejoin the battle later.

This does NOT say that one of your fighters must be in the territory.
My understanding of reading this is it kills everything in the territory, and that IF there are any of yours in the territory, they will also be killed.

The first time I got it I tried and failed to use it on a territory with only enemy fighters and wondered what was wrong.

You have copied the text exactly from the manual, but that is a poor English translation too.

Here is an English translation of the original French (a little wordy but very clear in meaning):

Choose one of your Basic or Mercenary Fighters. Remove them from the Battlefield as well as ALL the other opposing and allied Basic Fighters or Mercenaries present, without exception, in the same Sector. These Fighters are returned to their original bag and may eventually come back into play later.

Even what it says you select (a territory) is wrong in the current English text.
I'll set the above as an English -> English translation fix request, but you might want to clean that up.

Oops, should be 'territory', not 'sector' -- I've updated the translation fix request.