Group saving doesn’t work

What’s the problem:

1. I open the list of Ads and either use Quick Edit for a single ad or open the ad directly.
2. I select the group where I want the ad to appear.
3. After clicking Save/Update, I see that the selected group is correctly saved, but several random additional groups are also selected without me choosing them.

What I’ve tried:

• Cache/Optimization tests:
  • Disabled WP Rocket completely and trying changing options one by one (caching, JS defer, JS combine, file optimizations) — issue remained.
  • Disabled Jetpack — issue remained.
  • Disabled WP Code lite — issue remained.
  • Disabled All in one SEO — issue remained.
• Fresh reinstall:
  • Completely uninstalled Advanced Ads (with “delete data” option enabled) and reinstalled — issue remained.
• Browser and Form inspection:
  • Inspected the HTML of group selection checkboxes:
    • Name attribute was tax_input[advanced_ads_groups][].
    • Expected for WordPress taxonomies, not for normal meta fields.
  • Noticed that even after selecting groups, JavaScript querySelectorAll('input[name^="ad_group"]:checked') returned an empty result (no checked inputs).
• JavaScript patch attempt:
  • Injected custom JavaScript to manually fix checked attributes on checkbox change – Didn’t solve the issue
• Dummy taxonomy registration:
  • Registered advanced_ads_groups as a real taxonomy via functions.php.
  • After registering, group checkboxes disappeared (at least i tried :D)
• Tried to “play” with Advanced Ads options
  • I’ve tried all rational and non-rational things I can think off… nothing worked.
    

Can you please help me out?
URL to the site I’m having a problem with: https://balkanrock.com/

The page I need help with: [log in to see the link]