Are you still running classic SharePoint team sites? Microsoft have announced that they will upgrade classic team sites to modern team sites soon, in March/May 2020.
Only home pages that haven’t been customized will be updated. that means that if you didn’t add any new web parts to your team site then your site will be upgraded in or around May 2020.
So you might not have anything to worry about. But I could imagine that some of your team sites have an updated home page and other sites don’t have an updated team home page. This could mean that you end up with a mix of modern and classic. It might be time to check out your classic teams sites over the next few weeks.
If you are still running classic, it might be a good time to plan the move to modern.
For more information see also the message in your admin center