I ran into an issue with flatpak version of Kdenlive that it would render only the topmost V track if it was a simple still image.
Preview worked fine.
Luckily, someone in Kdenlive’s Matrix suggested that I use an appimage. I used my distro’s version and the final render was fine.

Other than that I had positive experience with flatpaks in general.

I haven’t tried it but doesn’t flatseal let you setup steam’s permissions to allow external/additional directories or mounts?
What’s stopping steam’s access to other directories?