>Another comment asserted HTMX can handle this, it just needs to be configured to do so. If that is the case, then I don’t see an actual issue.

You can, but it feels like you're sort of fighting the htmx library at that point. I do it, as it's the least bad option, but I generally find footguns with violating HTTP conventions (e.g., returning HTTP 200 for a bad request) or going outside the mainstream of a library (telling htmx that HTTP 400 can have a meaningful response body).