The setup-first-blocks parameter defines the length of the setup phase, in which the following parameters are not applied:
mining-diversity admin-consensus-admin admin-consensus-activate admin-consensus-mine admin-consensus-issue mining-requires-peers |
So the most likely explanation is that you have mining-requires-peers set to true, and more than one address has been granted the mine permission, but the node has no peers. So it stops mining at the end of the setup phase.