Almost 6 years later and still no fix 
Before anyone tries to link me to any of these
Please note, these DO NOT fix the problem on mobile. While UI looks clean and crisp on PC, testing on mobile still shows they black outlines. I believe this is a bug tho, as it did fix on mobile when I used PixelFix, and the UI was looking clean and crisp. But when I play tested the next day, it was back to the old ugly black outlines. Still same images, so I don’t know what’s going on with the rendering or something but ye.
This video was taken when I had fixed the UI with PixelFix. As you can see, the buttons and their icons don’t have the black outlines.
Then the next day I join and I’m greeted with the ugly black outlined UI again
The button images in studio are referencing the PixelFix images, so I don’t know if images get their quality ruined after a few hours after upload or what. But this bug still isn’t fixed 
making UI for mobile a near impossible feat sorry, I should say GOOD looking UI a near impossible feat
I have some further proof this is still an issue. Image1 shows robux icons taken before the pixelfix (with time stamp)
Image2 shows it after pixelfix (taken 2 minutes after previous image)
Then this last image shows me on a few hours later (3 hours after I did the pixelfix) and the images have reverted to black outlines
Can we please get some word that this issue is being taken seriously? Really hindering my games progress