MlConfigIndexMappingsFullClusterRestartIT test fails

[przemekwitek]

Build scan:
https://gradle-enterprise.elastic.co/s/viwr5wyw4bbl4

Repro line:

REPRODUCE WITH: ./gradlew ':x-pack:qa:full-cluster-restart:v7.15.1#oldClusterTest' \
  -Dtests.class="org.elasticsearch.xpack.restart.MlConfigIndexMappingsFullClusterRestartIT" \
  -Dtests.method="testMlConfigIndexMappingsAfterMigration" \
  -Dtests.seed=3B1FB3899C54A485 \
  -Dtests.bwc=true \
  -Dtests.locale=uk-UA \
  -Dtests.timezone=America/Argentina/Salta \
  -Druntime.java=11

REPRODUCE WITH: ./gradlew ':x-pack:qa:full-cluster-restart:v7.15.1#oldClusterTest' \
  -Dtests.class="org.elasticsearch.xpack.restart.MlConfigIndexMappingsFullClusterRestartIT" \
  -Dtests.method="testMlConfigIndexMappingsAfterMigration" \
  -Dtests.seed=3B1FB3899C54A485 \
  -Dtests.bwc=true \
  -Dtests.locale=uk-UA \
  -Dtests.timezone=America/Argentina/Salta \
  -Druntime.java=11

Reproduces locally?:
No

Applicable branches:
master

Failure history:
New failure, introduced by #77416

[elasticmachine]

Pinging @elastic/ml-core (Team:ML)