In beta 1.8 creative mode,you only in ground sprint,And then fly,can Sprint fly.
But this was fixed in tne past Release version, you can in sky double click Walk Forwards key,or click Sprint key,it's can all.
In 1.16.1 creative mode,double click Walk Forwards key,can't Sprint fly.Only click Sprint key can do it.
I am unable to reproduce this. Please provide a screenshot of your game while your F3 debug screen is enabled.