Index stuck in building

We have a collection with about 400k small documents. We ran a command to create a simple index on this collection db.collection.createIndex({"field": 1})

The index never finishes. When I look inside db.currentOp() I see the following:

   "waitingForLatch" : {
	"timestamp" : ISODate("2022-09-19T08:56:32.870Z"),
	"captureName" : "FutureResolution"

And mongo docs say

The waitingForLatch document is only available if the operation is waiting to acquire an internal locking primitive (a.k.a. a latch) or for an internal condition to be met.

But I don’t understand what it means.

Any ideas why the index build never finishes, or what I can do or check in order to understand?