Lets say I have two files file1.mp4 and file2.mp4.
I am trying to concatinate them with following,

First I write to inputs.txt :

file file1.mp4
file file2.mp4

ffmpeg -y -f concat -safe 0 -i inputs.txt -c:v copy -c:a aac output.mp4

I get warning:

[vost#0:0/copy @ 0x7fc9dbc10b00] Non-monotonic DTS; previous: 60261, current: 32215; changing to 60262. This may result in incorrect timestamps in the output file.

output.mp4 got generated but it has indeed caused timestamp mismatch as I see a long static video introduced at the end (ie still video but has audio)
So checking encoding I found

file1.mp4:

  Duration: 00:00:04.02, start: 0.000000, bitrate: 184 kb/s

  Stream #0:0[0x1](und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(tv, bt709, progressive), 864x504, 46 kb/s, 30 fps, 30 tbr, 15360 tbn (default)

file2.mp4:

  Duration: 00:04:30.97, start: 0.000000, bitrate: 166 kb/s

  Stream #0:0[0x1](und): Video: h264 (High) (avc1 / 0x31637661), yuv420p(progressive), 864x504, 35 kb/s, 2.41 fps, 2.41 tbr, 10k tbn (default)