I am testing but I can’t see any issue in the latest Grav + Admin in regards to adding new pages. Can you PM me on Slack your user/pages folder, to check why would that happen?
Sorry I have moved things around since my post. There were 4 top level menu items. One of them was causing the problem (set to numeric prefix disabled). 2 of about 10 drop down items had sub menu and some sub sub menu items. When I disabled these two drop down pages and re-enabled them the problem went away.
During all of this the menu editor some times reverted to a previous set of menu items (after disabling all the top level pages, the next level items from one of the drop downs appeared in the menu editor. This may be as it should be but it was this set that recurred after I had enabled one or two of the top level items again.
A point I noted a few days ago is that changing the order of the sortable pages by drag and drop in Pages-Advanced caused problems in the menu editor.
Another point is duplicate and triplicate sets of one or more pages appeared in the Manage Pages lists at the time. Deleting one page from the list would remove the other in the list.
I hope this helps in case there is something lurking.