This may happen for all merge fields or just some of the merge fields in the message.
We are currently evaluating this issue and will update this article when we have more information. We found this behavior to happen when using Internet Explorer, when drag/drop is used to add the merge field, instead of double clicking the merge field.
In the interim, either use a different browser, for example Chrome, or if using Internet Explorer, double click the merge field to add it instead of drag/dropping it.
Drag/Drop any merge field to add it, and then close the merge field window
Note: merge field is added as <img src="blob:F55B5728-180E-4803-9580-2B18E550A1BB"
Close the source code box
Click Insert>Merge Field
Double Click the merge field to add it
Note: merge field is added as expected: <img src="/insertField.field?id=57&nmode=0&name=Name&type=1" style="cursor: move;" title="Individual first name, middle initial, and last name - or organization name" data-runat="server" data-fieldid="57" data-attribid="0" data-searchable="-1" data-isloop="False" data-htmlencode="True" data-fieldtype="1" data-fieldname="Name" />