With a collapsed arch there is a fault at the base of the system.
A collapsed arch/ankle has the knee in an unstable position.

Have you watched the hip archetype videos within the 14-Day Mobility Challenge | 7 Green Lights?
The calcaneus can get stuck with a collapsed arch.

Pro Episode #3 – The Flat Feet Solution
One thing to do is to cultivate position. Don’t let your feet collapse. It takes time to develop.
An episode series which address rebuilding the feet and exercises to start incorporating.
Rebuilding Your Feet With Brian MacKenzie
Rebuilding The Feet, Part 2
Rebuilding the feet, part 3