Unfortunately trim with fill loop, as well as a couple of other things, aren’t intuitive and to the extent that there are bugs (there are) they’re really hard to nail down.
I followed your description above and not only does the same thing not happen, the ‘errors’ I get aren’t consistent. So sometimes I get what you get, then two minutes later I do the exact same thing and get a different result.
I reported problems with Trim a while back:
BUG (?)- Automation Trim in Preview - Nuendo - Steinberg Forums
And got little traction in that thread.
Sadly I get the feeling that Steinberg got to a point with the automation system where they basically said “That’s good enough” and left the rest alone, or at least downgraded the priority so much that it’s likely not going to change this decade.