I thought the containment vessels were designed for this sort of event (meltdown).
The 2 events surpassed the design parameters of the plant.
The containment vessel/building is 3rd part of a reactor’s defense in depth. The first is the fuel rod cladding, then the reactor pressure vessel, then the containment building.
It is the reactor pressure vessel’s integrity that would be of most concern as it holds the fuel rods, which have melted to varying degrees.
“I thought the containment vessels were designed for this sort of event (meltdown).”
Nope. A containment vessel is meant to contain the situation if the pressure vessel is compromised, say, if the melted fuel rods puddle in the bottom of the pressure vessel and then melt on through that. Not good if the containment vessel has been breached, because the next thing after that is dirt.