Please note that if you are building something against the 6.8 runtime, use only the latest Flatpak Builder 1.4.4.
Details
On Freedesktop runtime 24.08 we did some changes to Cmake libdir configuration that instructed apps to use /app/lib as the install path.
This was previously a patch to Cmake in Freedesktop SDK, but in 24.08 we dropped that patch and instead set CMAKE_INSTALL_LIBDIR in Flatpak Builder.