It's most probably the actual keys more so than anything else, if it's possible, you may be able to clean them up by clipping them off and seeing if there's any crud underneath, be careful if you do decide to flip them off.
Best phone camera 2017
Stunning new film posters by Hattie Stewart, Joe Cruz & more
iPad Pro 10.5in (2017) review
28 astuces pour profiter au mieux de votre iPhone