I discovered this glitch when I tried to recreate both forward and back buttons, since I don't have a Touch bar MacBook, I just ran the touch bar on a simulator and it works. I assume this glitch affects the real machine as well.
Simply put, since I can't find an icon that can provide the same crispness as the original, I decided to screen shot the one in the system, and use a third-party paint program to edit it. I set the size of the PNG to 60x60 (Which is the default height of the Touch bar), then pasting the original arrow on the button. After I dragged the PNG file to the icon, then take a screen shot, I found that all of the shades inside the arrow gone missing. Later I tried to edit it with a darker border, but after dragging them in, the same thing happened.
(As you can see, on the top are the ones with BetterTouchTool, compared to the original one, it's missing shades inside of both arrows.
You can see that the original PNG file have shading inside the arrow.
The same thing goes with "New Folder" button, it seems that the program simply ignoring when colors are close to white.
Device: MacBook A1181 (Mid-2009)
OS: High Sierra 10.13.5 (Bypassed)
BetterTouchTool version 2.530
Paint program: Paint S (You can check it out via App Store)