These are untouched video files created by a GoPro Hero 6 recording at 1080p50.
Each part is approx 4 GB and contains 11 minutes of material.
It's not necessary to download both parts for the issue to trigger. The same part can be added multiple times to the concat text file as shown in the example below.
ffmpeg.exe -f concat -safe 0 -i test.txt -c copy output.mp4
file 'GH010051.MP4' file 'GH010051.MP4' ...