not sure what operation on the pdf file, did you change appearence on signed field?
please do not change appearence when field was signed, this cause the digest data not match data changed before.
we has special version, which bigger than normal version, can sign the field with a cert file, in that method, you can customer appearence of field.
if you mean Acrobat draw a focus rectangle on a signed field, just after you click on the field.
i shall tell you, this behaviour is standard action of Acrobat.