Yeah, that’s a bit odd.
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linux-zenandlinuxwere updated to 5.8.7 on 2020-09-06 00:50 UTC -
linux-hardenedwas updated to 5.8.7 on 2020-09-05 18:36 UTC
Before that, linux-hardened lagged a bit behind linux and linux-zen:
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linux-hardened5.8.6 was built on 2020-09-05 00:22:59 UTC -
linux5.8.6 was built on 2020-09-03 17:27:39 UTC andlinux-zenjust a couple of seconds later.
So no, they didn’t skip 5.8.6 with hardened, they just hurried 5.8.7 it a bit, for some reason. Or linux and linux-zen got delayed a bit… 
I’m guessing it’s a coincidence and that people who are maintaining linux and linux-zen work independently from the person who maintains linux-hardened.