I ran into an interesting problem and would like to report it here in case others experience.
I had an HTTP call where JSON fields are case-sensitive. Saving the Flow in new designer would change some fields to either all caps or capitalize, causing the API call to fail. For example, I used field "id" in my HTTP JSON and it would change to "ID", and another renamed "comment" to "Comment".
It turns out I named one of my Flow Actions "ID" and another "Comment", and for some reason if a JSON name is the same as an Action except for capitalizing, it will rename it. MSFT is fixing this under 2412040040014930, but two workarounds include using old designer and ensuring Action names don't match JSON field (might be a good idea to use naming conventions for Actions so they don't match JSON fields or variables).
John