This is indeed the expected behavior, as documented here:
https://www./developers/blockchain-parameters/
The point of mining-requires-peers
is to prevent a node that becomes isolated from the network from going
ahead and mining its own irrelevant chain. You are right that mining-diversity also takes care of this, but there are some chains based on proof-of-work where there is no constraint on mining diversity.
The mining-requires-peers
setting is only applied if there is more than one address with mining
permissions, because in this case we assume these addresses will likely
be owned by multiple nodes. (Even if one nodes sees it has all these
addresses in its wallet, there could have been private key sharing.)