How did they sign it, exactly? Did you add a digital signature field to the file? If so, did you set it to lock other fields?
I have the same issue.
My client signed it with HelloSign. I have tried to sign with both Adobe Acrobat Reader and HelloSign when that didn't work. I can add my signature, but when I save it the signature disappears. I've tried to save it to the cloud, no success with the signature saving.
BUT - when I go to open the file on my phone, I can for a moment see all of the signatures right where they should be, until the page finishes loading then they disappear.