I face the same problem and have also found this related thread on the developer community. We face this issue after moving a work item to another team project.
Here they advise to run 2 database commands, i have not been able to test this yet. Also not sure if this is fixed in the latest Azure DevOps patch.
EXEC prc_SetRegistryValue 1, '#\FeatureAvailability\Entries\WorkItemTracking.Server.AllowAccessToDeletedAttachments\AvailabilityState\', 1
Based on the command they suggest to execute, it seams the work item attachment has been flagged as "deleted". Not sure why that happens though.