Here's the screenshot: (The beginning few seconds, the remainder was mono already so it didn't change)
https://pasteboard.co/JcsgT83t.png
On the right is the SAME Mp3 file in stereo.
On the left, the tracks have been merged into mono. Then they were exported through Kdenlive and opened with audacity.
It doesn't make much sense why the same Mp3 merged into mono would lose a huge amount of the sound amplitude! I've been struggling with this issue for a while now.

So to test this bug, just take any MP3 in your video and Mix the tracks to mono with audacity, and then re-load it in Kdenlive, you should get much lower audio volume.