mongoose schema optimisticConcurrency option on subdocuemnt

Question

I'm trying to implement optimisticConcurrency on mongoose Subdocument and i cant get to make it work.

consider

const SchemaA = new Schema({

    keyA: String,

    subdocA: new Schema({
                 subdocA_a: String,
                 subdocA_b:String 
             },{ optimisticConcurrency: true })
})

I wish to be able to update subdocA.subdocA_a only if subdocA.subdocA_b did not change (i dont mind if keyA changed).

i followed the docs on how to implement optimisticConcurrency here, without any success.

when i moved the optimisticConcurrency option to the parent Schema i.e:

const SchemaA = new Schema({

    keyA: String,

    subdocA: new Schema({
                 subdocA_a: String,
                 subdocA_b:String 
             })
},{ optimisticConcurrency: true })  <---------------

it worked. but i want the Concurrency control ONLY on the subdocument.

so i want to know if optimisticConcurrency can work on subdocument (mongoose docs are not clear about it), and if so, how to make it work.