ffmpeg concatenates two videos but the output video has a little white glitch after the end of the first one. How to correct?

Question

I'm trying to merge two videos into one using ffmpeg.

Here's the command I use:

ffmpeg -f concat -safe 0 -i files.txt -c copy output.mp4

Where the files.txt has:

file '/Users/user.name/Desktop/Part1.mp4' file. '/Users/user.name/Desktop/Part2.mp4'

The output video is fine, except for a little momentary white glitch after the end of the first video. How to I correct it?

P.S. I used to use this utility on an ubuntu machine previously and it worked like a charm always. This is the first time I'm using it on a mac (installed ffmpeg using homebrew).

P.P.S. I tried again in the old ubuntu machine, got the same glitch there too.

Many thanks in advance.