Arrow keys don't always work for key shortcut reassignment

I was trying to reassign the left and right arrow keys to “Change Brush: Decrement Size” and “Change Brush: Increment Size”. They reassign fine, but in practice, it doesn’t seem like they work. I’ve reassigned other keys to this function without issue.

Interestingly enough, arrow keys also seem not to function with assignments such as Rectangle Tool, but the tools that require you to hold it down like Rectangle Tool (quick) work fine.

I’m using Steam, it says v1.2.6-x64 in the About, and I use Windows 8.1.