[CI] ThreadPoolMergeSchedulerStressTestIT testMergingFallsBehindAndThenCatchesUp failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic-platform-support #9449 / windows-2025_checkpart1_platform-support-windows
• elasticsearch-periodic-platform-support #9316 / windows-2025_checkpart1_platform-support-windows

Reproduction Line:

gradlew ":server:internalClusterTest" --tests "org.elasticsearch.index.engine.ThreadPoolMergeSchedulerStressTestIT.testMergingFallsBehindAndThenCatchesUp" -Dtests.seed=CB96BF2E3345B05 -Dtests.locale=csw -Dtests.timezone=Antarctica/Davis -Druntime.java=24

Applicable branches:
9.0

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: 
Expected: is <157>
     but: was <12>

Issue Reasons:

• [9.0] 2 failures in test testMergingFallsBehindAndThenCatchesUp (0.4% fail rate in 498 executions)
• [9.0] 2 failures in step windows-2025_checkpart1_platform-support-windows (10.0% fail rate in 20 executions)
• [9.0] 2 failures in pipeline elasticsearch-periodic-platform-support (10.0% 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-distributed-indexing (Team:Distributed Indexing)