I've created a Survey123 Connect form from an existing feature layer. I had to add a signature field to that feature layer. When I created the report for the survey, the signature just shows up as the file name. I have looked at the Report Quick Reference and it indicates the field will return a text value and not an image.
To test outside of all my form mechanics, I created a new form from the feature layer and only changed the signature field to "image" and "signature", and then published it. The sample report shows only a filename and the Quick Reference for this new report only shows the signature field will return a text value. This makes me think it has to do with the field definition in the feature layer.
I'm really confused about the connection of images to Arc On-line. We have another feature layer that has images and when I look at the fields for that layer I see a column for "Photos and Attachments" which is a type "Attachment". One would think that a survey123 field would be specified with the same data type, but that isn't an option when you create a field in the feature layer.
Signature questions are just images with a special interface. You can't get text data out of them and they require a feature layer with attachments enabled as that's where the image is stored. If you need a name with the signature you'll have to make another text question in your form.
I figured it out with help from a co-worker. You do NOT add a field in the feature layer. As soon as I assigned a different name in the Excel spreadsheet, new signatures show up in the Data tab in Survey123 on line and in any reports I generate.