The only thing that witnesses do is to confirm that they saw the testator sign the will. They do not have to see the contents of the document (or even to know that it's a will).
So, as far as I see it, the problem must relate solely to the witnessing of your grandfather's signature. For example, if your grandfather's will is dated (say) "23rd June 1997" (and apparently signed by him on that date) and a witness has added the date "24th June 1997" after his own signature, it would suggest that the witness was not actually present when your grandfather signed the will. That would lead to the Probate Registry initiating enquiries to try to ascertain whether the witness simply got the date wrong (which would probably not prevent probate being granted) or whether your grandfather presented the witness with a pre-signed document (which would invalidate the will).
Chris