I guess this is another reason why the general consensus is you shouldn't store attachments in Access tables. Better to store the path to the file and make use of that instead. IIRC, there is a system table where you might find that these objects are being kept. Perhaps it is re-populating your field with those objects. Not sure that the usual responders here will know because I don't think any of them/us use such fields as a rule.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.