Both of my kids have flat feet. For them, it was something genetically passed on by yours truly. It's much more pronounced with the older one. Both her and my arches totally collapse while standing.
This is an unfortunate (but workable) condition for a gymnast. It's very unnatural for her NOT to run and bounce on her heels. When she runs (during vault especially), you can distinctly hear the pounding on the runway. When she tumbles, she struggles more than others to get the same height. Her knees are also slightly knocked when relaxed. That leads to sloppy form (more so on bars and vault) which she also worked very hard to overcome. Her feet still involuntarily separate during her clearhip handstands and sometimes her giants.
Her ex-HC was the first to notice the condition at L4 and how it was impacting her performance in form and execution. Over the year in gymnastics, the pounding has been lessened but it still can be heard more than others'. One thing that helped was to jump rope regularly. This exercise built her muscle memory to stay off the heels and use the balls of the feet (for the most part).
Because it's a condition she was born with (and she does gymnastics mainly for fun), we chose not to seek medical remedy for her flatfootedness (and deal with podiatric aids). For other active sports and activities where she puts a ton of weight and needs to be quick on her feet (such as hiking and ski racing), she uses OTC insoles/footbeds. Luckily, they appear to be enough to correct her over-pronation and knockknees during those times.