LiveCycle ES4, preview pdf will not test the electronic signature. All other fields work and this field will work if the document is saved then opened with reader and signed. I have Adobe Acrobat Pro DC, Acrobat DC Reader and have ran updates on this software.
Sorry for the delay in response to your query and apologize for the inconvenience caused.
Please refer and try the steps from the following KB docs which discusses the similar issue LiveCycle Designer can't add digital signatures or save (Sign and Send PDFs) Link: https://answers.acrobatusers.com/LiveCycle-Designer-add-digital-signatures-save-q292858.as px
Hope this helps, and let us know how it goes.