cracked jockey wheels - Ultegra R8000

in Workshop
I recently noticed my jockey wheels have small cracks radiating out from the centre. Both pulleys, similar size cracks. They are on a Ultegra R8000 mech with about 8000km on them. I haven't seen anything like this before. I suspect they weren't about to fail anytime soon and it did not stop me from riding the next day. But I did manage to find replacements (the same). It'll be interesting to see how the new ones hold up.
I can't find any examples of this online (at least in the last few years). Anyone seen this? Otherwise they have lots of life left. I don't think I had ever removed them from the RD for cleaning. I do have a habit of staying in my big chainring unless it is big/big - perhaps that loads them up a bit more than typical. By comparison, my XT M8000 drivetrain is well worn, the T pulley is well on its way to being a disc, and they are otherwise fine. Interestingly, the Shimano website lists R8000 jockey wheels as being compatible with mtb rear mechs, but not the other way around - perhaps an oversight on the spec sheet.
Any thoughts on the cracks?

I can't find any examples of this online (at least in the last few years). Anyone seen this? Otherwise they have lots of life left. I don't think I had ever removed them from the RD for cleaning. I do have a habit of staying in my big chainring unless it is big/big - perhaps that loads them up a bit more than typical. By comparison, my XT M8000 drivetrain is well worn, the T pulley is well on its way to being a disc, and they are otherwise fine. Interestingly, the Shimano website lists R8000 jockey wheels as being compatible with mtb rear mechs, but not the other way around - perhaps an oversight on the spec sheet.
Any thoughts on the cracks?

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Are the DA any better though?
I'm on the verge of needing some new jockey wheels for my winter bike that currently have 105 & was gonna get some Ultegra which I know will horrify you but the 105 never seem to last that long, for me anyway.
DA seems a bit overkill for a winter bike though.
What makes them better?
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Marin Nail Trail
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