[CI] MixedClusterEsqlSpecIT test {union_types.MultiIndexIndirectUseOfUnionTypesInSort SYNC} failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic #5745 / 8.11.4_bwc
• elasticsearch-periodic #5740 / 8.11.4_bwc
• elasticsearch-periodic #5735 / 8.11.4_bwc

Reproduction Line:

./gradlew ":x-pack:plugin:esql:qa:server:mixed-cluster:v8.11.4#javaRestTest" -Dtests.class="org.elasticsearch.xpack.esql.qa.mixed.MixedClusterEsqlSpecIT" -Dtests.method="test {union_types.MultiIndexIndirectUseOfUnionTypesInGrok SYNC}" -Dtests.seed=F3E77CDB633752CD -Dtests.bwc=true -Dtests.locale=om-ET -Dtests.timezone=America/Metlakatla -Druntime.java=23

Applicable branches:
8.x

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: Expected an empty list
0: <unexpected> but was "Field [@timestamp] cannot be retrieved, it is unsupported or not indexed; returning null"

Issue Reasons:

• [8.x] 3 consecutive failures in step 8.11.4_bwc
• [8.x] 3 failures in test test {union_types.MultiIndexIndirectUseOfUnionTypesInSort SYNC} (2.7% fail rate in 113 executions)
• [8.x] 3 failures in step 8.11.4_bwc (100.0% fail rate in 3 executions)
• [8.x] 3 failures in pipeline elasticsearch-periodic (100.0% fail rate in 3 executions)

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

[elasticsearchmachine]

Pinging @elastic/es-analytical-engine (Team:Analytics)