This is a regression – in 4.27.2 the bracket keys do not change the Brush Size even though it is the default hot key. However, changing the hot key to something else works in 4.27.2.
Hot Keys are not working for Mesh Painting Brush Size. If the default '[' ']' keys are removed as a hotkey for everything else but mesh painting it still doesn't work. Changing the Hot Key to something different also doesn't work.
Brush size is increased or decreased as expected.
Grid Snapping is changed. Brush Size does not.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-151461 in the post.