Can you post your form?
attached you can find the form/ data reduced and anonymized to it's mere core, so you should be able to reproduce the issue.
You just have to click the "sign 1st sign" Button.
When the form is loaded without a signature the sfctr.init() function is called.
The "sfctr" coding handles the navigation within the form as well as the "late rendering" functionality. As the form consists of a header part (with the signature objects) and 1 to n following sub pages, each of those page is "rendered" (build up with data) not until the page is navigated to.
If you have any question don't hesitate to ask.
Best you also have a look at the click event of the "sign 1st sign" Button.
I looked at the form, the cause of the problem is somewhere in your script (and there is a lot of it!)... unfortunately, I can't determine the exact cause. I would suggest that you look script that is either changing data or the form template in some way as this could cause the signature to be invalidated.
Hope this helps.