This is an old version of the kernel. Do you see the same behavior onSadly, RedHat is not very keen on updating their kernels. I did a quick experiment on Ubuntu box with kernel 4.4, and indeed it does not behave this strange way. So I'll write this off as a long fixed kernel bug, stick to auto-renicing in cron, and look forward to upgrading this CentOS machine some day. Thank you very much for your help.
a newer version of the kernel? (4.8 is the latest stable kernel)