The “design flaws” are:

Reactor design with active boiling and positive void coefficient, such that your power profile is essentially inverse to your normal control rod position. Additionally you are severely impacted by things like trips of a reactor coolant pump.

No mechanical limits on location of the graphite followers (not just control system limits, but a physical hard stop)

The graphite followers having to move past the fuel is the result of the two above plus the operators making some very dumb decisions.

If those graphite followers were never removed as much as they were, if they essentially stayed in the lower portion of the core, they would have been fine. But when you pull them out far enough, then during the reactor trip they will initially add reactivity.