When a trial judge is found to have erroneously failed to hold a competency hearing before trial and must hold one long afterward, who has the burden of proof?

The California Supreme Court today held that the burden of proving incompetency is on the defendant, consistently with the statute for pretrial competency hearings. The opinion in People v. Ary, No. S173309, notes that a majority of jurisdictions agree, although there is an Eleventh Circuit opinion contra.