Hey there. This looks like a rollup issue, but it actually turns out to be a bug in the composite aggregation (which rollup uses). If the composite agg is iterating over a field in one index, then moves on to the next index in the search and finds that the field is unmapped in that new index... it accidentally mapped that field as numeric. This would throw an exception when the original field was a keyword like in your case, because the after key can't be cast to a numeric.
There's a bugfix to correct this behavior. It just merged and should be available in 7.1: https://github.com/elastic/elasticsearch/pull/41280
Since this is caused by unmapped fields, a temporary fix would be to go add the correct mapping to the indices missing the field(s), or just wait for 7.1 to land.
Sorry for the hassle!