The source packages are in the Debify repository so you can always do
apt update
apt source ffmpeg
to see the exact source it was built from. Patches are in the debian/patches directory of any package.
Make sure it really downloads the source from debify and not the regular package from Buster.
In this case, I can point you to the
patches on Gitlab as well, there is also a copy in the ffmpeg trac issue. I had to tweak it to resolve the compiler errors that are mentioned in trac, the version in Gitlab compiles without errors.