Fix for the 2018 MBP, probably others but make sure you get the right keys for other models.
Had exactly the same issue, holes appearing in the black coating on some keys. For me it was Left Shift, A, and Left Command. Keyboard cleaned regularly, can't see any reason for it other than manufacture issue, I expect contamination on the tile before the black coating is applied.
Being a bit obsessive about my machine I looked for a solution and found this:
OLVINS Replacement Key Cap Set UK English Keyboard Keycaps Keys for MacBook Pro Retina 13" 15'' A1989 A1990 UK keyboard keys 2018 2019
www.amazon.co.uk
Full set of key caps costing £21 in the UK, similar must be available elsewhere.
Replacement is very easy. Thin blade at top edge of key cap and lift, I did it a corner at a time, but smaller keys just pop off completely. In theory, once the top of the key if lifted just slide the key towards the top to take out (bottom is on 2 hooks), but once both clips are undone it will probably come off of it's own accord. To fit the new key (just the cap, if your replacement came with the butterfly attached just remove it), slide the new key in towards the front of the key space at about 20-30 degrees to engage the bottom hooks, then push down at the top to clip in place, you'll hear a couple of clicks.
As I said, I have three replacement caps fitted, they're indistinguishable from the originals.
Image shows swapped out keys, so you can see the replacements in situe. And the back of one of the replacements, maybe you can zoom in and see the hooks at the bottom and clips at the top.
One note, I've read that the space bar is difficult to change and can break - I haven't had to yet, but it looks like there are 2 sets of hooks to undo.