I promise you, it is not a "legal issue." And if it ever was, Trump v. US was clear that it is not.
If you are talking about a forgery coming out of the office of the president: the pardon stands, and the forger is subject to criminal liability. That is, if some staffer signed a pardon with Biden's name, and Biden never knew anything about it, the pardon still stands. The courts are not going to peer into the executive branch. It's the same way that a pardon received through a bribe stands, even if the bribe could be proven. Again, if there was any doubt, the Supreme Court laid it to bed.
If you are talking about a forgery that did not come from the president's office: well, it's a forgery and it has no legal effect, full stop.
This is a decided issue. I'm not talking about it any more. If you need to create for yourself some weird identity as a "person who knows about law" despite not having studied it, practiced it, read about it, read judicial opinions and statutes -- do what you're going to do. I've explained the law clearly and succinctly and I won't do it again. You can deny or accept. Your choice.