The issue is being caused by the Formidable Form. I created a test signature form and it works.
You can see from the “Entries” screen that the form is not saving the signature image because it’s missing, then if you open the “Sig test” you’ll see the signature is saved and visible.
I created a duplicate of your form for testing and it appears the signature field works if you move it to the last page of the Formidable form. So Formidable Form is failing to save the signature if the signature field is on the first page, I didn’t try any other pages except the last as we are way out of the plugin area of support… At this point you can open a ticket with Formidable, pay us to open a ticket with Formidable, or move the signature to the last page… (or test other pages…)
We remain at your service!