SERVER-45624 introduced this parameter because it also made the balancer split the sessions collection into 1024 chunks by default.
SERVER-45624 our tests never created enough sessions for the initial chunk to get split, so they always ran with 1 chunk. SERVER-45624 set the parameter to 1 in servers.js, which applies to all ShardingTest-based suites, but we should also set the parameter to 1 in ShardedClusterFixture-based suites.
This should be backported as far as
SERVER-45624 is backported.