[CI] ThreadPoolMergeExecutorServiceDiskSpaceTests testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-intake #24393 / part1
• elasticsearch-pull-request #76939 / part-1
• elasticsearch-pull-request #76927 / part-1
• elasticsearch-pull-request #76914 / part-1
• elasticsearch-periodic-platform-support #8926 / windows-2019_checkpart1_platform-support-windows
• elasticsearch-periodic-platform-support #8926 / windows-2025_checkpart1_platform-support-windows
• elasticsearch-periodic-platform-support #8926 / windows-2022_checkpart1_platform-support-windows
• elasticsearch-periodic-platform-support #8926 / ubuntu-2004-aarch64_checkpart1_platform-support-arm
• elasticsearch-periodic-platform-support #8926 / amazonlinux-2_platform-support-aws
• elasticsearch-periodic-platform-support #8926 / amazonlinux-2023_platform-support-aws

Reproduction Line:

./gradlew ":server:test" --tests "org.elasticsearch.index.engine.ThreadPoolMergeExecutorServiceDiskSpaceTests.testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges" -Dtests.seed=7F5F34048EA948B0 -Dtests.locale=se-SE -Dtests.timezone=Kwajalein -Druntime.java=24

Applicable branches:
8.19

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: 
Expected: a collection containing <WARN: [indices.merge.scheduler.use_thread_pool] setting was deprecated in Elasticsearch and will be removed in a future release. See the breaking changes documentation for the next major version.>
     but: mismatches were: [was <CRITICAL: [indices.merge.scheduler.use_thread_pool] setting was deprecated in Elasticsearch and will be removed in a future release.>]

Issue Reasons:

• [8.19] 2 consecutive failures in test testEnqueuedMergeTasksAreUnblockedWhenEstimatedMergeSizeChanges
• [8.19] 2 consecutive failures in step part1
• [8.19] 2 consecutive failures in step part-1
• [8.19] 2 consecutive failures in step windows-2019_checkpart1_platform-support-windows
• [8.19] 2 consecutive failures in step windows-2025_checkpart1_platform-support-windows
• [8.19] 4 consecutive failures in step windows-2022_checkpart1_platform-support-windows
• [8.19] 3 consecutive failures in step ubuntu-2004-aarch64_checkpart1_platform-support-arm
• [8.19] 6 consecutive failures in step amazonlinux-2_platform-support-aws
• [8.19] 2 consecutive failures in step openjdk23_checkpart1_java-matrix
• [8.19] 3 consecutive failures in step openjdk17_checkpart1_java-matrix
• [8.19] 3 consecutive failures in step ubuntu-2404_platform-support-unix
• [8.19] 3 consecutive failures in step almalinux-8_platform-support-unix
• [8.19] 3 consecutive failures in step sles-12_platform-support-unix
• [8.19] 5 consecutive failures in step rocky-8_platform-support-unix
• [8.19] 2 consecutive failures in step oraclelinux-7_platform-support-unix

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)

[alex-spies]

This is failing 50% of the time and affects backports to 8.19. I'll mute this in a moment.

[albertzaharovits]

Fixed in #129980