Is the field in question an attachment field?
When uploading via the API it takes a while for an attachment to become fully attached. First the url of the attachment is set, then the file data is uploaded to Airtable’s servers, and the thumbnails are generated. Sometimes the process breaks down if there is a problem with the url or communication.
Perhaps this process can also sometimes take a while with a native form uploading an attachment. I’m just speculating here, but it could explain what you see. Maybe someone from Airtable will chime in. If not, you can ask customer support.