This is a key missing piece to many people’s hip mobility work.
There are two primary phases to muscles: Concentric (shortening) actions and eccentric (elongation) actions.
It’s common to see concentric hip mobility-focused work where people are doing exercises actively working against resistance to get into a given range of motion.
There's lots of information on what to do if you have tight hip flexors, but what a lot of it misses is the difference when issues are only on one side: