sharepoint online - Check out not required but created files are in check out status

Question

Today, for the first time, I watched an anomaly behaviour in all my SPO sites (all in classic experience).

I have Document Libraries where it's not required documents to be checked out, until yesterday when I created new documents they were in "check in" status but today they are created in "checked out" status.

To be exact, the first document I create in a Document Library is in "check in" status, all the further documents in "check out" status: what happened? How can I create every time documents in "check in" status?

Update: I have done some tests but I have to give you some more information.

I have a custom Content Type for Document Set and one for files present in the Document Set. Two properties of the Document Set are Shared Columns with the Content Type of the files: if I set these properties as hidden I can create a file in "check in" status. My problem is I need these properties are visible.

Answer 1

Check if Library Settings -> Version Settings -> Require Check Out set to Yes:

The set above will make file check out after uploading files in library.