FAA does require the manufacturer to proof that the aircraft can continue flying if the reverser is deployed in flight. In many aircrafts, this certification is done in flight.
In the case of 767, the real test was not done and the design was accepted based on calculations. In real scenario, however, unknown to the designer, the exhaust from the reverser was blown over the wing, causing the wing to lose lift. That, in turn, cause an excessive roll which has recovery window of only 4 seconds.
The certification calculation did not include the effect of the jet exhaust that cause lost of lift. The effect was found only after the accident when Boeing simulated the reverser effect in wind tunnel.
That implies that in flight reverser deployment in a 767 means death if not recovered within 4 seconds (deflect full rudder and aileron in opposite direction).
The cause of the accident was pointed at the interlocking circuitry of the reverser. Under certain failure mode, the electricity can leak through a circuit and cause in flight reverser deployment. The circuit has been redesigned.