Starting from v4.2, disabling the balancer is also resulting in disabling the autosplitter.
When a ShardingTest is initialized with enableAutoSplit: true, the balancer is disabled and autosplitter is enabled.
Those 2 operations are not happening atomically, so there is a small window in which both components are disabled. This makes it possible for the v4.0 ShardingUptimeReporter to kick in and assume - until next refresh in 10 seconds - that autospliting is disabled.
When that happens, autosplit.js fails in the sharding_last_stable_mongos_and_mixed_shards suite between v4.2 and v4.0 .
Both v4.2 and v4.0 are flagged as affected, but the fix should target just one version depending on how it will be implemented: fixing the test in v4.2 (more likely) or the code in v4.0.