use case - merging two indexes into new output.
the flow is to take 2 indexes, both having timestamp and pivot by time where aggregations are unique to each index. KPI1 - index1 and KPI2 - index2.
when using ES 7.6.1 as long as the pivot keys are same no issues.
when using ES 7.8.1 this actually no longer works the same transformation. as workaround we found that both indexes have to identical from mapping perspective on the aggregations fields. otherwise it will not work.
basically the main issue is that this is not smooth upgrade if you have running transformations with merging indexes.
my kind suggestion is to bring back the previous functionality as in 7.6.1