[CI] ThreadPoolMergeExecutorServiceDiskSpaceTests testMergeTasksAreUnblockedWhenMoreDiskSpaceBecomesAvailable failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic-platform-support #8818 / rocky-8_platform-support-unix
• elasticsearch-periodic-platform-support #8794 / rhel-7_platform-support-unix
• elasticsearch-periodic #8749 / openjdk17_checkpart1_java-matrix
• elasticsearch-pull-request #75663 / part-1
• elasticsearch-pull-request #75592 / part-1
• elasticsearch-periodic-platform-support #8716 / oraclelinux-8_platform-support-unix
• elasticsearch-pull-request #74922 / part-1
• elasticsearch-pull-request #74898 / part-1

Reproduction Line:

./gradlew ":server:test" --tests "org.elasticsearch.index.engine.ThreadPoolMergeExecutorServiceDiskSpaceTests.testMergeTasksAreUnblockedWhenMoreDiskSpaceBecomesAvailable" -Dtests.seed=B68EBECF5BF88B65 -Dtests.locale=ie -Dtests.timezone=PLT -Druntime.java=24

Applicable branches:
8.19

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: 
Expected: is <164L>
     but: was <155L>

Issue Reasons:

• [8.19] 8 failures in test testMergeTasksAreUnblockedWhenMoreDiskSpaceBecomesAvailable (1.0% fail rate in 801 executions)
• [8.19] 4 failures in step part-1 (4.2% fail rate in 96 executions)
• [8.19] 3 failures in pipeline elasticsearch-periodic-platform-support (15.8% fail rate in 19 executions)
• [8.19] 4 failures in pipeline elasticsearch-pull-request (4.3% fail rate in 93 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)

[elasticsearchmachine]

This has been muted on branch main

Mute Reasons:

• [main] 3 failures in test testMergeTasksAreUnblockedWhenMoreDiskSpaceBecomesAvailable (0.6% fail rate in 522 executions)
• [main] 2 failures in step part-1 (1.1% fail rate in 183 executions)
• [main] 2 failures in pipeline elasticsearch-pull-request (1.1% fail rate in 182 executions)

Build Scans:

• elasticsearch-periodic-platform-support #8796 / debian-12_platform-support-unix
• elasticsearch-pull-request #75382 / part-1
• elasticsearch-pull-request #75276 / part-1