As you mentioned, the tags have not yet been added to the downstream devices, causing KEPServerEX to pull them from the upstream device, resulting in an error. To prevent this, you may need to disable the missing tags in Kepware. However, this can be a time-consuming task if there are a large number of tags, while it would be quicker if the count is smaller. The effort required depends entirely on the number of missing tags in the device.
So to do so:
KEPSEerverEX Administration Utility > Settings >Security Policies>Permission Assigned to Anonymous Clients (Expand) > Browsing (click arrow head) > Custom permission > Choose device and expand it further till you select tag , click arrow head > Deny