Hi… It sounds like you might unknowingly have two machines, in which case each would inherently have an independent and unsynchronized volume.
(Two is the generic platform default, to avoid data loss when a physical host machine fails, —among other surprises.)
What does fly m list show?
Aside: It’s best to avoid using volumes directly, if you can, since they are more of an advanced (or at least intermediate-level) feature on the Fly.io platform. If you just need key-value storage, for example, then Tigris is much easier.
(On the other hand, if you only require a single, read-only file, then you can separately place it manually on each volume, via --select.)