https://pypi.org/project/flatbuffers/25.12.19/#files contains only a wheel, and is missing a source distribution. Previous releases did have a source distribution, see for example https://pypi.org/project/flatbuffers/25.9.23/#files.
It is bad to not have a source release available for multiple reasons, including breaking workflows for distro packagers and preventing usage by dependency analysis tools. Please consider uploading a new sdist for that release.