Jackthemimic on Nostr: How exactly would the condition arise where a core or knots node upgrades to BIP110 ...

How exactly would the condition arise where a core or knots node upgrades to BIP110 with the legacy chainstate then doesn't immediately have to reorg to the BIP110 valid chain? How is this any different than if I mine a block to my node, don't broadcast to any peers then reconnect a day later? Is that also a "consensus bug?" Or is it just reoganization?