The Professional Liability Insurance which the architect/firm carries will protect all parties even if/after the architect leaves the firm.
The policy will continue to protect all parties involved in the project (including the architect who no longer works at the firm), at least from a professional liability angle. The architect will still need to participate in the legal proceedings of the claim even if they leave the firm because they will be a party to the claim. The architect may still be found negligent and can still lose their licensure for stamping a project at a prior firm.