Generate a video with same settings as an existing video in order to join them

[NailBomber]

I have a H264 video to which I want to attach a little blank video.
I would like to avoid re-encoding a video and just remux it instead.
I tried different settings using ffmpeg x264 to generate a compatible blank video and got a result with seemingly same parameters (as reported by ffprobe and MediaInfo).
However when trying to join both videos MKVToolNix complains that "The codec's private data does not match".

I actually pretty much gave up on the idea, but before doing that wanted to hear from pros on this forum.

I downloaded a trial of CodecVisa to get more information, but it doesn't show anything in 'Stream Summary'.