I always use Kdenlive to creates videos on my YouTube channel, I have used different distros like Ubuntu, Manjaro, KDE Neon etc, and didn't have any issue with Kdenlive Flatpak version.
Yesterday I switched to Linux Mint, Installed Kdelive Flatpak version, but the window is not resizable and unable to full screen, even I press ctrl+shift+F it won't full screen/stretched/fitted. There is no maximize button in the tittle bar, only minimize and close button.
Usually this only happens on PPA or Appimage version, but weird that this also happened on flatpak version but only in linux mint.

https://i.postimg.cc/Gh69BMKZ/kden.png

https://i.postimg.cc/Gh4Hm0FF/kden2.png

Have you tried posting to a Linux Mint forum or mailing list, since it seems specific to that distribution?
Also, have you tried some method of installing other than flatpack? If the distribution package works on Mint, but not the flatpack, then the problem is not inherent to Kdenlive, but to that particular flatpack package.