I, too, had this problem with the app. In my case, I am prompting the user for a 17 digit project number and my fear is that if the user inputs their project number wrong the first time they will always enter it wrong because of the autocomplete. I don't want to turn autocomplete off entirely (I don't believe you can) because it can be a great help for users, assuming they entered it right the first time.
Here's what I have found - there is no setting within the Flow app to clear history or anything similar. Instead, go to your device's "app info" menu (I'm using Android), then select Flow and find storage -> clear data. Select that. When you return to the app you will be prompted to login again, but thankfully the autocomplete entries will be gone.
Simply clearing the cache does not create the desired result.