Rollup Job splits array field

I have an index where I store an array filed together with other fields:

        "_index" : "my-index",
        "_id" : "xxx",
        "_score" : 1.0,
        "_source" : {
          "Timestamp" : "2022-10-05T09",
          "Action" : "Performed",
          "Genre" : "sport",
          "SubGenre" : [
          "Count" : 2

And I have an ISM policy to rollup this index

"rollup": {
              "ism_rollup": {
                "target_index": "my-index",
                "description": "Rollup job",
                "page_size": 1000,
                "dimensions": [{
                  "date_histogram": {
                    "source_field": "Timestamp",
                    "fixed_interval": "1d"
                  {"terms": {"source_field": "Action"}},
                  {"terms": {"source_field": "Genre"}},
                  {"terms": {"source_field": "SubGenre"}}
                "metrics": [
                    "source_field": "Count",
                    "metrics": [

(I’m including only the rollup part)

My problem is that the SubGenre fields get split by the rollup job, and this messes up my Count, because what was supposed to be just a document with the value Count=2 now becames three different documents with value Count=2. I Sum the value of Count and instead of having a value of 2 I now have a value of 6 from my rolled up index, while from my original index the sum of Count is 2.

Is there a way to keep the array together in the rollup job? I read a bit around and the only solution I found up until now is to split the array field in N field e.g SubGenre_1,SubGenre_2…SubGenre_N, but I would like to avoid this, because I don’t know the number of element that can be in the SubGenre field.