Mine seem to be fine in all the colours I have them in... usually white and trans colours have
seen more cracking in my experience, but my cheese slopes haven't been used a hell of a lot.
Have you used these ones much? do you push down hard? do you remove from the groove or do you tilt the brick against the stud from the wider edge? I'm not suggesting it's your fault in the slightest, the pieces SHOULDN'T be splitting like that - ABS should be far more hard wearing in terms of life/longevity (these pieces were only introduced in 2004) so even given their maximum age, they should still be fairly free from fatigue... just interested to know what sort of usage the cheese slices have seen and whether certain techniques result in more fatiguing than others.
I've got similar sized pieces that have been in my collection for three or four times that long and aren't showing anything like the same problem... although I did have to replace both Luke and Leia's arms in my 7190 Falcon in around 2007 - for similar looking splits near the hands... perhaps there is an issue with thin cross-section material in white.
ABS in it's raw state is semi-translucent ivory tending toward white, so perhaps it is a chemical-make-up issue for all white pieces in thin cross-section. IIRC from my materials technology lectures, pigments and other such additives will affect the durability properties of the final product in the same way as the heat of the plastic when injected, the heat of the mold, cycle times of the injection mold machine and whether there is any cooling of the mold between cycles.
So many factors could be at play, I doubt you can make a case for replacement as they are used
pieces, but I think it's important that TLC be made aware of such a serious issue regarding the life and quality of their product.
Having said all that, I did find a lot of my old (25yr+) 1 x 1 trans-red plates had split in half - but I was a biter* as a child.
Perhaps we should all be keeping our eye on the cheese slice!
*don't hate me, I'm rehabilitated now