Hi… It sounds like you might unknowingly have two machines, in which case each would inherently have independent and unsynchronized volumes.
(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.)