it isn't set to allow, but it isn't set to deny either... so it should be fine, i tried to allow it tho, nothing changed. To be honest it's really strange... i tired to do another 1.16.5 server but it doesn't work... so i guess it's not strict to 1.17.1
SynchedEntityData is for syncing things to the client, not for saving to disk. If you don't need the data on the client, use a plain field, not SynchedEntityData.
For saving to disk you need to override addAdditionalSaveData and readAdditionalSaveData.