cache instances being ignored

I get this when writing to a very large RocksDB table, from many connections
[Note] Detected table cache mutex contention at instance 1: 28% waits. Additional table cache instance activated. Number of instances after activation: 2.
2018-12-25 20:19:00 51 [Note] Detected table cache mutex contention at instance 2: 20% waits. Additional table cache instance activated. Number of instances after activation: 3.

but my configuration is

-----------------------------------------------+----------+

| rocksdb_force_compute_memtable_stats_cachetime | 60000000 |

| rocksdb_table_cache_numshardbits               | 19        |

| table_definition_cache                         | 400      |

| table_open_cache                               | 2000     |

| table_open_cache_instances                     | 24        |

+------------------------------------------------+----------+

I get those errors almost right away. I insert records from 64 connections using GNU parallels