Microsoft rolled out a change that now opens all lists in SharePoint in Microsoft Lists instead. I am not a fan. You can however run a Powershell script to put them back in SharePoint or use the Navigation Elements. Here’s an excerpt on how from Microsoft MC600726 · Published 21 Jun 2023 · Last updated 16 Aug 2023 :

How this will affect your organization:

Lists with the new experience will always open inside the Lists app, whether those lists are opened from Lists Home, a sharing link, a direct link, or a link in SharePoint navigation. This means SharePoint site elements like the header and left navigation will not be shown on initial load. There are two ways to show these site elements in a List:

• Users can click the “Open in Site” button in the upper right corner of the list. This causes the List to reload inside the parent SharePoint site with those site elements visible on the page.
• Admins can set a property on a site collection to make all lists in that site collection always load with the site elements intact. This property is called ListsShowHeaderAndNavigation. This an example PowerShell cmdlet that will set this property for a site given its URL: Set-SPOSite -Identity https://contoso.sharepoint.com/sites/site1 -ListsShowHeaderAndNavigation $true

Site owners can navigate to the Navigation Elements settings page and configure a new setting called “Lists in Sites.” This setting causes navigation links to lists in that site collection to always open inside the parent SharePoint site.

(I don’t see this setting in our tenant yet, but this is what we are supposed to see).

What you need to do to prepare:

The ListsShowHeaderAndNavigation setting is currently available if you wish to proactively set this setting to prepare for this rollout. Doing so will mean that lists inside those sites will always be shown with SharePoint site elements like the header and left navigation.