I literally did this twice last week!

After starting a new playthru recently, the first time this raid happened I went out to try kill them. It wasnt working out and so I kited them a bit away from my main base and then ran to another. I never saw them again.

A 2nd raid of the same type, this time I killed one and saw the other turn around and run off! I was annoyed actually because he was nearly dead but ran into the ocean - never saw him again.

So either its mods thats doing this (because I am using plenty), or is this a recent change - because without enemy mods on my first playthru just some months ago I also did this.