KASSERT_WARN() is used for now since the impact is not known.
This is based on the patch and report from https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=193696
A problem I know of is that malloc(9) calls uma_zalloc(9) and due to the non-panic KASSERT_WARN, it shows the same message again. It would not make sense to only place the KASSERT_WARN only in uma_zalloc_arg(9) for now as then memguard and redzone would be missing the check.