Delprado, Delta flight 191 of 1985 showed that convective weather that was traversable by the preceding aircraft rapidly became deadly for a wide body three miles in trail.
Assuming that it is acceptable because some aircraft had flown the route, and we do not know how long before that was, is not a good strategy when there is significant convective weather.The US NWS give the life cycle of a thunderstorm as thirty minutes. The previous aircraft may have seen it whilst it was still building as cummulus, and not the weather shamrock saw.
I believe that a groundstop was put in place after the event.