The Kotlin coroutine driver Flow methods do not implement first rather they rely on the built-in coroutine first* extension methods.

When working on https://jira.mongodb.org/browse/JAVA-5666, we found findOne works as expected per its requirement everywhere, except for kotlin-corountine, as batchSize = 64 was sent out. So when conflicts with SyncFindIterable's inherited method `first` from SyncMongoIterable returns element instead, as below (in SyncMongoIterable):
 

override fun first(): T? = runBlocking { delegate.firstOrNull() }

For this reason, SyncFindIterable's first() method won't hit  the underlying FindPublisher's first() method as in other modules, which would emulate findOne behaviour including batchSize()  and basicBatch options.