ignore_sticky_posts generating a wrong behaviour in WP_Query

[SirLouen]

Bug Report

Description

According to the docs:

ignore_sticky_posts (boolean) – ignore post stickiness (available since version 3.1, replaced caller_get_posts parameter). false (default): move sticky posts to the start of the set. true: do not move sticky posts to the start of the set.

Something is going wrong with ignore_sticky_posts behavior when certain WP_Query args are being used (the __in ones more specifically).

More importantly, since, ignore_sticky_posts is false by default, it can be easily reproduced as soon as we use any of these arguments for the query.

Environment

• WordPress: 6.9-alpha-60093-src
• PHP: 8.2.28
• Server: nginx/1.27.5
• Database: mysqli (Server: 8.4.5 / Client: mysqlnd 8.2.28)
• Browser: Chrome 135.0.0.0
• OS: Windows 10/11
• Theme: Twenty Twenty-Five 1.2
• MU Plugins: None activated
• Plugins:
  • Test Reports 1.2.0

Steps to Reproduce

1. Install this simple plugin for testing: ​https://raw.githubusercontent.com/SirLouen/wp-query-tester/refs/tags/1.0.1/wp-query-tester.php
2. Create a second username
3. Create some posts in this order:

• First post, make sticky
• Second not sticky post. Call it exactly, "Not Sticky 1", with slug "not-sticky-1".
• Third, make sticky post
• Fourth not sticky post
• Fifth a sticky post, with second username as author
• Sixth, not sticky with second username as author

3. Create another post and add the short code [wp_query_tester]
4. 🐞 Bug occurs

Expected Results

• Query 1: All posts with sticky on top
• Query 2: All posts ignoring the stickyness
• Query 3: All posts from Author 1
• Query 4: The post with slug "not-sticky"

Actual Results

1. Query 1 is ✅

• Query 2 is ✅
• Query 3 is ❌. Queried only author 1. A post from author 2 appears (because its sticky)
• Query 4 is ❌. Queried just the slug not-sticky-1. All sticky posts appear, the last one is the one queried.

Additional Notes

This topic is seriously correlated with this issue #63307 just as a reference.

Supplemental Artifacts

Plugin repo: ​https://github.com/SirLouen/wp-query-tester/

Screenshot Sample