As part of the task https://jira.mongodb.org/browse/SERVER-112726 the random balancing was enabled in buildscripts/resmokeconfig/suites/sharded_collections_jscore_passthrough_with_config_transitions_and_add_remove_shard.yml suite.
With this new configuration array_index_and_nonIndex_consistent.js started to fail.

New configuration:

fixture:  
  class: ShardedClusterFixture  
  enable_balancer: true  
  random_migrations: true  

The test uses isIxscanMultikey() helper function from jstests/libs/query/analyze_plan.js, which calls getPlanStage() to retrieve plan stage information. However, getPlanStage() only returns the plan stage from the first shard in the indexPlan array.

When the ContinuousAddRemoveShard hook adds a new shard:

• The new shard (e.g., shard-rs1_1) is added to replace an old one
• This new shard generates a query plan with:

"isMultiKey" : false,  
"multiKeyPaths" : {  
    "val" : [ ]  
}  

• Initially, if this shard appears at the end of the shard list, the test passes because getPlanStage() reads from the first (existing) shard
• After the balancer performs random migrations, the new shard can be reordered to the top of the shard list
• When this happens, getPlanStage() now returns the plan from the new shard (with isMultiKey: false), causing assertion failures

The test makes an incorrect assumption that checking only the first shard's plan is sufficient in a sharded environment with dynamic shard topology changes.