[CI] MixedClusterClientYamlTestSuiteIT test {p0=synonyms/110_synonyms_invalid/Reload index with invalid synonym rule} failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic #9140 / openjdk21_8.17.9_java-matrix-bwc
• elasticsearch-periodic #9119 / openjdk17_8.17.9_java-fips-matrix-bwc

Reproduction Line:

./gradlew ":qa:mixed-cluster:v8.17.9#mixedClusterTest" -Dtests.class="org.elasticsearch.backwards.MixedClusterClientYamlTestSuiteIT" -Dtests.method="test {p0=synonyms/110_synonyms_invalid/Reload index with invalid synonym rule}" -Dtests.seed=B5AB7763F5983DA7 -Dtests.bwc=true -Dtests.locale=haw-US -Dtests.timezone=CNT -Druntime.java=21

Applicable branches:
8.19

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: Failure at [synonyms/110_synonyms_invalid:143]: 
Expected: "bar"
     but: was "foo"

Issue Reasons:

• [8.19] 2 failures in test test {p0=synonyms/110_synonyms_invalid/Reload index with invalid synonym rule} (2.4% fail rate in 83 executions)
• [8.19] 2 failures in pipeline elasticsearch-periodic (50.0% fail rate in 4 executions)

Note:
This issue was created using new test triage automation. Please report issues or feedback to es-delivery.

[elasticsearchmachine]

Pinging @elastic/es-delivery (Team:Delivery)