What PDF reader are you using? Some readers don't work properly with PDF forms.
By default we use Acrobat Reader, but we have tried it in PDF Xchange as well and it has the same results
Is the form there if you view it in Adobe Acrobat Pro?
It is exactly the same if viewed in Adobe Acrobat Pro.
I have attached a screenshot of what we see, don't know if this makes a deference but it looks as if the whole form has been set to read only on submission as you can't add or change anything.
As you can see you can see the outline of the signature it just looks as if it has been signed in white rather than black. Oh have check that this was not the case.
You could try restoring your InDesign preferences because somehow InDesign didn't apply the form field. Then try it again.
But you could also use the Form panel of Acrobat Pro (Tools > Forms > Edit) and add the Signature field in Acrobat Pro. I'd say the latter is faster and surer.