Although the PTO is permitted to spend its already collected fees. It isn’t actually able to spend fees collected since the shutdown. My understanding is that the Office is in cost-cutting mode based upon President Trump’s statement that the shutdown could last for “weeks or even years.”
Once funds run out, the PTO is looking for ways to keep the doors open enough to continue to accept new application filings — while shutting down examination and PTAB activities. It is likely that a special PTO funding bill would receive bipartisan support — keeping the patent office rolling while other government services are shut-down.