I have SP 2019 farm, the document library migrated from MOSS 2007 is showing classic page even "New Experience" enabled in List Settings
After some research, I found some other web part other than XSLTListView is added to that page view. after removed those webparts page working like a charm.
I have followed below steps to find out the issue:
- To get page rendering type (REST type) copy and page below rest end point in browser (update server host name): http(s)://server/_api/web/getfilebyserverrelativeurl('/sites/dev/ECMTest/Forms/AllItems.aspx')/pageRenderType
- check the response value: I got value 1, then checked with the previous Reference URL i got description about the problem (MultiWebpart)