If you look at Fukushima disaster, it was not a design problem either, but maintainence problem. It was probably running more without modifications than it was designed for.
There were updates suggested to modernize the facility, but for cost cutting purposes they were ignored:
It was a design problem and a maintenance problem. A lot of the design decisions made in that era later proved to be Very Bad Ideas, like how there was no proper hydrogen containment above the reactor vessel.
Newer designs have suffered more major faults and managed to contain virtually all of the radiation. American designs, in particular, place great emphasis on having an extremely resilient containment structure above the reactor. A lot of things can go horribly wrong but so long as the extremely radioactive gas is contained it can later be cleaned up. These radioactive elements are extremely toxic, but also very short lived. You just need to buy time.
The Fukushima design may as well have had a tin roof, it exploded almost immediately and exposed the reactor to the elements. If that's not a design flaw, I don't know what is.
That and a number of the systems necessary to keep the reactor under control depended on poorly positioned generators that weren't flood-proofed. This seems like a major oversight on a building located in a tsunami and typhoon zone.
There were updates suggested to modernize the facility, but for cost cutting purposes they were ignored:
https://m.phys.org/news/2011-03-iaea-japan-nuclear-quake-wik...
It always comes down to cost cutting when the accidents are rare enough.