An error when updating indexes

When choosing to update index via the right click on a collection, the automatic script created when partialFilterExpression is involves drops the $exists condition. For instance, I have this index:

{

uuid: 1,

"public.offerNumber": 1

},

{

name: "uuid_1_public.offerNumber_1",

partialFilterExpression: {

"public.offerNumber": {

"$exists": true

}

}

}

but the autoscript I mentioned looks like that:

db.StatesHistory.dropIndex("uuid_1_public.offerNumber_1");

db.StatesHistory.createIndex(

{

uuid: 1,

"public.offerNumber": 1

},

{

name: "uuid_1_public.offerNumber_1",

partialFilterExpression: {

"public.offerNumber": {

: true

}

}

}

)

as you can see the "$exsits" key is missing, and an error is indicated.