Shard allocation with hot - warm stacks

[Jathin]

All nodes(master, hot and warm) are on elasticsearch v7.2.0.
hot nodes have node.attr.hot_stack: hot_stack_a and warm nodes have node.attr.warm_stack: warm_stack_b

_cluster/settings
  {  "transient": {
        "cluster": {
            "routing": {
                "allocation": {
                    "include": {
                        "warm_stack": "warm_stack_a",
                        "hot_stack": "hot_stack_a"
                    }
                }
            }
        }
    }

i am adding a new set of hot nodes with node.attr.hot_stack: hot_stack_b and then update settings

_cluster/settings      
  {  "transient": {
        "cluster": {
            "routing": {
                "allocation": {
                    "include": {
                        "warm_stack": "warm_stack_a",
                        "hot_stack": "hot_stack_b"
                    }
                }
            }
        }
    }

it seems elasticsearch is not relocating all shards to hot_stack_b and no shards are allocated to warm_stack_a.
so when i do another cluster settings update cluster.routing.allocation.include.warm-stack : "warm_stack_something_else" and then update it back to "warm_stack_a" (not make any changes to cluster.routing.allocation.include.hot_stack setting) then all shards move to wam_stack_a and no shards allocated to hot_stack_a or hot_stack_b