Noticed this happening quite a few times now, my target highlight will sometimes disappear once another enemy I'm killing dies.
I've think this has been happening since some time in DF S3~4 though so it's not a new occurrence.

Example:
https://streamable.com/zcx5fb

I'm also having this problem since the War Within began

I think this is related to soft-targeting, i.e., a bug I introduced when I added that. Basically, similar to this here: #528.

Looking into the code, I might have found the issue, but I need to test if my fix works.

I just uploaded a new beta version that might fix this issue. Any feedback if you still see the bug would be appreciated.

This is hopefully fixed now in version 12.0.10