Are you in a film project (35mm or 16mm)? Those projects have “hard subclips” in order to protect metadata that may be different – although that was more of a Keykode protection issue with early film workflows and transfers and is less of an issue in non-film based workflows.
But the “hard subclip” concept still remains in a film project preventing a user from trimming past the boundary of its subclip start-end. If not a film project, it may be something else but I would have to test some scenarios to know for sure. I tend to work in film 35mm projects to have 1/4 frame sync capabilities with translation to Pro Tools when turning over sound.
Michael