The most likely explanation is that mining has stopped, because of this combination of events:
- Your blockchain has reached the end of the 'setup phase' (defined by the setup-first-blocks blockchain parameter).
- Your blockchain has been configured with mining-requires-peers=true and you have not overridden this setting at the node level with the miningrequirespeers runtime parameter.
- More than one address has been granted mine permissions.
- Your node is not connected to any others (see getpeerinfo output).
If so, this is the expected behavior. It is designed to prevent a single node that becomes disconnected from the network from running away with solo mining. You can fix the problem by changing any one of the above conditions, now or for future blcokchains.