As far as I am concerned, this is not a widespread issue, if you can, please post or send me some pictures of your coins with faded firstbits for analysis.
What I can say is that sometimes, depending on the letters comprising the Firstbits it gets printed a bit blurrier, having a much smaller font size, also sometimes with the hologram on top it affects readability as window is not 100% transparent. What I make sure is that the private key letters are all complete, firstbits are just extra information.
See:
https://i.imgur.com/0QI8rPN.jpgHere is a picture of a 2014 Dogecoin:
https://i.imgur.com/W2iVjhv.jpgFirstbits have not faded there, I have been using the same ink and process for the last six years. For what I have seen in my collection, some are "barely readable" since the beginning. (Having released the coin list should not make that a problem)
Furthermore I have my own "Canary", a couple tubes of 2014 10k Dogecoins + 2014 10k Bits BTC stored in humidity of 60%+, which I open and redeem from time to time to ensure the key is still readable, no issues so far.
I want to be as open and transparent about this as possible, and verify if some firstbits are indeed fading, as far as I know no private key has "suffered" from this issue. But bear in mind that no process, of any maker, is bulletproof and there will always be risks with physical keys.
Any doubt, please let me know.