Anglican Orders have been invalid since the time of Edward VI (not Henry VIII; until his death, the Church of England held on to Catholic beliefs and practice; all bishops and priests ordained during his reign received valid Holy Orders).
In the reign of Edward VI, a new rite of ordination was promulgated, which explicitly omitted, on the basis of Protestant belief, the office of offering the Mass as a Sacrifice—the core function of a priest. The Catholic Church judged this to be a defect of intention. Even though the ordinal was latter corrected, the break occurred at that time.
It’s not the priest’s belief that confects the Eucharist. It’s his Holy Orders.
That said, this is not to say the Eucharist as celebrated by Anglicans is devoid of grace. It does not have the Real Presence of course, but this act of commemorating the Lord’s Supper is still at least a sacred, devotional act and can and does impart actual graces to those disposed. In many cases, these actual graces have moved Anglicans to reconcile with the Catholic Church.