Thank you thank you.
I just modified my user mount script line from
docker run -v /mnt/user/appdata/other/rclone/mergerfs:/build --rm trapexit/mergerfs-static-build
to
docker run -v /mnt/user/appdata/other/rclone/mergerfs:/build --rm -e TAG=2.36.0 trapexit/mergerfs-static-build
and all is working again.