Just had a field research flee

Check your inventory. You probably caught it.

I believe it’s a glitch and you should actually have it if you check your pokemon. Sort by recents.

You can also see it flee if you're locked out of catching pokemon either by softban (Which is caused by GPS teleports), speed cap (traveling faster than 80km/h or so) or hitting the catch limit (3500-ish pokemon per week).

In all of these cases, the encounter will still be accessible in your encounter queue later when the game updates you to a state where you can catch pokemon again.