Comment 1 for bug 1993716

juju uses the constraint tags that are prefixed with "pod." or "anti-pod." or "node." into pod affinity selectors.
your constraints are: --constraints="tags=mldatanode=true,^mlgpunode=true"
The tag keys don't have the required prefixes to be translated into affinity selection expressions.

A contrived example.

juju deploy somecharm --constraints="tags=node.foo=a|b|c,^bar=d|e|f,^foo=g|h,pod.foo=1|2|3,^pod.bar=4|5|6,anti-pod.afoo=x|y|z,^anti-pod.abar=7|8|9"

would result in

kubectl get -o json statefulset.apps/somecharm | jq .spec.template.spec.affinity
{
  "nodeAffinity": {
    "requiredDuringSchedulingIgnoredDuringExecution": {
      "nodeSelectorTerms": [
        {
          "matchExpressions": [
            {
              "key": "bar",
              "operator": "NotIn",
              "values": [
                "d",
                "e",
                "f"
              ]
            },
            {
              "key": "foo",
              "operator": "NotIn",
              "values": [
                "g",
                "h"
              ]
            },
            {
              "key": "foo",
              "operator": "In",
              "values": [
                "a",
                "b",
                "c"
              ]
            }
          ]
        }
      ]
    }
  },
  "podAffinity": {
    "requiredDuringSchedulingIgnoredDuringExecution": [
      {
        "labelSelector": {
          "matchExpressions": [
            {
              "key": "bar",
              "operator": "NotIn",
              "values": [
                "4",
                "5",
                "6"
              ]
            },
            {
              "key": "foo",
              "operator": "In",
              "values": [
                "1",
                "2",
                "3"
              ]
            }
          ]
        },
        "topologyKey": ""
      }
    ]
  },
  "podAntiAffinity": {
    "requiredDuringSchedulingIgnoredDuringExecution": [
      {
        "labelSelector": {
          "matchExpressions": [
            {
              "key": "abar",
              "operator": "NotIn",
              "values": [
                "7",
                "8",
                "9"
              ]
            },
            {
              "key": "afoo",
              "operator": "In",
              "values": [
                "x",
                "y",
                "z"
              ]
            }
          ]
        },
        "topologyKey": ""
      }
    ]
  }
}