Ferromagnetic materials have a hysteresis, a stickyness to their field... materials with large hysteresis are used to make permanent magnets, and those with very low hysteresis are best for transformer cores, because each transition through hysteresis represents loss. All cores have it, it is an engineering optimization to choose a "hard" (high hysteresis) or "soft" magnetic material.

If you want to store a bit, as in core memory, or magnetic logic, you'd use a "hard" material. It is likely the cores in the TROS module were "soft".

If the hysteresis is low enough, even the normal atomic motion due to heat can cause the field to dissipate over time.