The other witnesses would flag this block as a bad-block and reverse it
No, since RCs are not part of consensus, this would be a perfectly valid block and would be included on the blockchain just like any other block. It really wouldn't create much of a problem either, the block size is limited anyway so at worst some accounts would be able to get in some more transactions than they normally would have been able to in any blocks created by that witness - which is a function of the amount of witness votes they have.
interesting and good to know.
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Witnesses can reject a consensus-valid block if they want, and it's still a soft fork. From a consensus perspective it looks the same as the situation where a block is received too late due to network delays and gets ignored.
The only thing witnesses can't do with a soft fork is accept a consensus-invalid block.