Good day Michal,
Just to gently clarify before going any further, this forum is separate from Microsoft Support. I don’t have the ability to access or investigate individual accounts because this is a user‑to‑user support forum.
Since Search this list relies on both search indexing and security trimming, direct access to an item doesn't always guarantee that it will appear in search results. The behavior suggests a mismatch between the owner account's search security context and the permission information currently stored in the search index, particularly because the list uses folders with unique permissions.
Please try the following:
Option 1: Check whether the issue is specific to the owner account
Sign in with another Site Owner or Site Collection Administrator account.
Open the same list and use the same view.
Search for the same item using the same keyword.
Test the affected account in an InPrivate/Incognito window.
If available, test in a different browser.
If another owner can search successfully, the issue is isolated to the affected account. If all owners experience the same behavior, the issue is related to how search is processing the folder permissions.
Option 2: Verify Item-Level permissions
Open the list.
Go to Settings > List settings > Advanced settings.
Under Item-Level Permissions, confirm that Read access is set to Read all items.
Make sure the list isn't configured to allow users to read only items they created.
If Read all items is already selected, no changes are required.
Option 3: Perform a permission comparison test
Select an affected folder and note its current permissions.
Temporarily create a test folder that inherits permissions from the list, or restore inheritance on a test folder.
Create a new test item in that folder.
Allow time for search indexing to process the item.
Test Search this list again using the affected owner account.
If the item is searchable in the inherited folder but not in folders with unique permissions, the issue is associated with how search is handling those unique permissions.
Option 4: Contact Microsoft Support
If the issue persists after completing the tests above, I recommend opening a support request with Microsoft Support through the Microsoft 365 Admin Center > Support > Help & Support for further backend investigation.
Turn off the "Support Assistant" feature.
Use the keyword "Support" to quickly bypass automated suggestions > Select Contact Support.

In the ticket description, please clearly describe the issue, including any error messages and the steps you’ve already tried. You can also attach screenshots or screen recordings.
Additionally, sharing your preferred contact times may help the support team handle the case more efficiently.
A technical support engineer can perform a remote session to investigate the situation, verify the backend configurations, and run any necessary synchronization tools to resolve the problem. If the issue requires further attention, they can escalate it to a specialized team for deeper analysis.
If you have any questions, please don’t hesitate to reach out at any time.
Thank you for your patience and understanding.
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