The changelog of freenginx already reflects changes not noted in the nginx changelogs. For example the mime type updates.
The fork from mainline to stable also happened several days before nginx on freenginx.
The even versions are stable. Every year in April the mainline (uneven) version is forked to the new stable branch. The old branch is retired at this time and receives no further bug fixes.
All changes introduced in 1.25.x are now in the stable release, 1.26.
In comparison with 1.24 (stable) HTTP/3 support has been introduced and several fixes.