Microsoft Lists UI in SharePoint List UI instead of modern SPO UI

Karol_Kozowski Are you using targeted release or standard release tenant? If targeted release tenant, it maybe the temporary issue at Microsoft end.

I don't have this issue on our SharePoint tenants (I tested on 3 tenants). When I open the SharePoint list from SharePoint site, it opens with the SharePoint view (with site navigation, etc).

If you are opening the list saved in SharePoint site from "Microsoft Lists" app/site (Eg.https://contoso-my.sharepoint.com/personal/gsanap_contoso_com/_layouts/15/Lists.aspx), 

then it is default behavior to open the list with "Microsoft Lists" UI and not SharePoint UI with site navigation, etc.

Follow below steps to open the SharePoint list in SharePoint view:

1. Go to your SharePoint site.
2. Click on Settings (gear) icon from top right corner and select Site contents.
3. Locate your SharePoint list and click on list name. It should open the list in SharePoint view

Also, make sure you are not using this query string parameter while opening the SharePoint list: env=WebViewList

If you use this query string parameter with the SharePoint list URL, it open the list with "Microsoft Lists" UI.

If the issue still persists, I will suggest you to raise a support ticket with Microsoft directly and report this behavior: Get M365 Support - Online Support 

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