Cosmetic filters randomly ignored #3403

Prerequisites

• I verified that this is not a filter list issue. Report any issues with filter lists or broken website functionality in the uAssets issue tracker.
• This is NOT a YouTube, Facebook or Twitch report. These sites MUST be reported by clicking their respective links.
• This is not a support issue or a question. For support, questions, or help, visit /r/uBlockOrigin.
• I performed a cursory search of the issue tracker to avoid opening a duplicate issue.
• The issue is not present after disabling uBO in the browser.
• I checked the documentation to understand that the issue I am reporting is not normal behavior.

I tried to reproduce the issue when...

• uBO is the only extension.
• uBO uses default lists and settings.
• using a new, unmodified browser profile.

Description

Lately (last 2-4 weeks) I've been noticing some inconsistencies in the way cosmetic filters are applied (or rather not applied). They are sometimes randomly ignored and elements that have been already blocked are sometimes visible.

This happens on various websites, so it's definitely issue of uBlock, rather than website changing layouts. It most often happens right after creating new cosmetic filter, that after refresh the filter gets ignored, less often it happens with filters that are older.

I should mention that I'm a heavy user, using uBlock to save some screen space from visual clutter on websites I visit often so I currently have over 2600 rows in my filters (and counting). Could that be that uBlock doesn't work well with filters of this length?

A specific URL where the issue occurs.

dell.com
reddit.com
cyberark.com

Steps to Reproduce

1. Go to a website
2. Create cosmetic filter
3. Observe element disappear
4. Refresh
5. Observe element being back

Expected behavior

Elements being blocked consistently.

Actual behavior

Elements being blocked inconsistently.

uBO version

1.59.0

Browser name and version

Firefox - 130.0.1 (64-bit) - flatpak

Operating System and version

Linux 6.10.10