There was a somewhat similar recent thread here which contains some useful diagnostic tips/info, as well as linking to other threads.
As in that thread, might be useful to share output of
GET /_cat/nodes?v&h=name,role,disk.used_percent,disk.used,disk.avail&s=role
If at any instant there was no indexing ongoing how can the cluster know there will be no indexing in the next instant/second/minute/whatever? I guess if your indexing only happens at very specific and predictable times, you could code something, but ... seems wrong. Is the reallocation actually causing you / your clients some actual issue? Or is it that it's just unexpected. And, is your cluster stable, nodes are not semi-frequently leaving/re-joining the cluster?