x264: Ensuring cfr with lavf/ffms input

I'd like to double-check that I've correctly interpreted how to ensure that x264 uses cfr when using lavf or ffms as the demuxer, because they make x264 vfr aware, resulting in vfr encoding by default (at least with the inputs that I'm feeding it). Specifying the frame-rate results in cfr encoding:

x264 --demuxer lavf/ffms --output /dev/null --fps 24000/1001 ~/input.mkv

According to this wiki, --fps implies --force-cfr, so I shouldn't need to set that explicitly. What I want to establish is that the above ensures that vfr never kicks in at all -- in other words, that my video has not gone cfr->vfr->cfr internally, which might conceivably cause problems. So far, my encodings have completed without discernible frame-rate issues, but I'd like to be absolutely sure that I've not overlooked/misinterpreted something.

I'm on Ubuntu 19.10.