Wrote it off as my failing to notice something the first time it happened this game, but on the second one I was able to reproduce the issue on-demand. After engaging in a manual battle in which neither side was defeated, my unit appeared a significant distance away from the engagement and the combat did not continue another round.

You can make out the unit as the bottom-right unit in the final screenshot, adjacent to another enemy unit instead of the one it engaged. Sometimes the unit will appear to have its previous health levels, but selecting the unit confirms its HP values match.

If I then take the other unit in the area and engage another enemy unity, I get a similar result.

Both units survive combat, and suddenly my unit appears all the way northeast at a distance 6 tiles from where it started. I can no longer see the enemy unit due to Fog of War. It also appears to have its starting health, until you select the unit.

I've been able to replicate several times, through relaunching the game and reloading the save. Issue is specific to Manual Battle; if I choose Instant Resolution, the unit behaves as expected.

Attached the save file for reproduction. Manual Battle Issues.ctr