This was discovered through reworking a test case. I noticed the bug when I was hitting crtl Z through all of my edits and noticed that the brush size value did not change even though the notification said it was undid. This also occurs when you change and undo Tool Settings as well.
Result - A toast notification pops stating "Undo: Edit Brush Size" but the Brush Size Value does not change
Expected - The Brush Size changes back to the original value when Ctrl Z is pressed
Tested in 4.12.5 - the bug is present therefore not a regression.
There's no existing public thread on this issue, so head over toAnswerHub just mention UE-36568 in the post.