Hi Daniel,
Do you have the same layers referenced in the same project?
Yes
Is all your data in the same projection as the basemap? Is your basemap designed to come offline does it need to be sideloaded?
Yes
Do you have any custom symbology?
No
Are you creating offline areas on the fly (from the mobile device) or through the Field Maps Designer?
On the fly
Have you tried using one layer at a time to isolate if it is a particular layer causing the issue?
Yes, still can't isolate to one layer.
Do you have enough space on your device for offline areas or have you exceeded the limit of 16 offline areas?
Yes
How are the items in your group shared are they are all shared under the same level ie, owner, organisation,public.
All shared with owner + with relevant group
Thanks for getting back to me, I will raise a support case. Just want to raise strong frustration with offline working in Field Maps, I have come across multiple posts with the same error and there has been no fix by Esri. It's extremely frustrating for things to break without the creator of the map amending anything and users out in the field being affected by the loss of important data.