A property is either consequential or it isn't. If it is then it must be at the very least [1] observable. If it is not, then you just have an imaginary distinction. The instability of an unstable system is something that can be observed if important. Stability is part of "behaviours and outcomes".

It's baffling that people cannot see the obvious contradiction in simultaneously asserting a system is missing a crucial property whose effects cannot be observed.

[1] It can be invisible. The important part is that its effects are observed.