[CI] SearchHitsTests testConcurrentSerialization failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic-platform-support #8422 / almalinux-8-aarch64_checkpart1_platform-support-arm
• elasticsearch-periodic-platform-support #8404 / almalinux-8-aarch64_checkpart1_platform-support-arm
• elasticsearch-periodic-platform-support #8386 / almalinux-8-aarch64_checkpart1_platform-support-arm
• elasticsearch-periodic-platform-support #8380 / almalinux-8-aarch64_checkpart1_platform-support-arm
• elasticsearch-periodic-platform-support #8338 / ubuntu-2004-aarch64_checkpart1_platform-support-arm
• elasticsearch-periodic-platform-support #8308 / ubuntu-2404-aarch64_checkpart1_platform-support-arm

Reproduction Line:

./gradlew ":server:test" --tests "org.elasticsearch.search.SearchHitsTests.testConcurrentSerialization" -Dtests.seed=7B2C4DC9DE6E6426 -Dtests.locale=en-TK -Dtests.timezone=Etc/GMT-13 -Druntime.java=24

Applicable branches:
8.19

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: 
Expected: <org.elasticsearch.search.SearchHits@1c0f5553>
     but: was <org.elasticsearch.search.SearchHits@1c0f5553>

Issue Reasons:

• [8.19] 6 failures in test testConcurrentSerialization (0.8% fail rate in 795 executions)
• [8.19] 4 failures in step almalinux-8-aarch64_checkpart1_platform-support-arm (20.0% fail rate in 20 executions)
• [8.19] 6 failures in pipeline elasticsearch-periodic-platform-support (30.0% fail rate in 20 executions)

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

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[elasticsearchmachine]

Pinging @elastic/es-search-foundations (Team:Search Foundations)

[cbuescher]

This looks very much related to #127971