It’s really not flatten brush at all, like Zbrush 3D or coat 2021.
I really don’t know what brush it is and what is it’s use 
It’s producing more volume than flattening, strange such obvious issue has not yet been fixed.
(i tried with Dynotopo off, but the brush behaves the same)
Heh, I’m not sure what Flatten is supposed to do either, but Scrape is probably the right brush for this effect.
Thanks, Blender “Scrape” brush is in fact a Flatten brush.
I don’t know why it is named “Scrape” instead of Flatten.
And i don’t knwo what is purpose of Blender named “Flatten”.
Flatten should be used with “Orignal Normal” and maybe “Original plane” turned on. Otherwise it will update the projection while painting.
