The mergeIndexes method in CoreAdminRequest class accepts an array of srcCores, but it only merge the last core of array into targetCore. Related code as follows,
The for-each loop above overrides the SRC_CORE value in params and only the last one reserved when loop ends.
We should remove the for-each loop and set SRC_CORE value with an array of srcCores.
The code above is in CoreAdminRequest class line 330