Apple advises not to store its titanium credit card in leather.
The titanium card of Apple is glossy and smooth, but one might need to treat it delicately if one expects it to look the same as the day it was purchased.
Apple has also posted a cleaning guide for the physical credit card that warns against storing it in leather and denim, if stored it could lead to the card discolouration that would not wash off.
Many leather wallets have got linings and plastic cards have their own issues; however, this indicates that it is more delicate than some would like it to be.
As per the same guide video, the card should also not be placed in direct contact with other credit cards or loose items in case it gets scratched on the titanium surface.
If there is some stain on the card, that could be cleaned off, using a microfiber cloth moistened with isopropyl alcohol.
This is not some operational issue of the credit card. It is not like that one’s card would stop working just because it is a bit dirty and one could have an incentive on using the Apple Pay more frequently regardless.
Having said that, titanium cards are more about flaunting status as it is used for making purchases. It is hard to give the vibes when the card looks as if it has been treated sourly.