Last item in navigation menu won't stay put–is there a limit?

I have added the last item to my top navigation menu and saved the menu but it changes location. When I first add it to the menu, it is identified as a page and inside the “Original location” is shown. I indent it under its parent page. When I click Save Menu, it outdents to the far left and “page” becomes “custom” and the “Original location” is blank. I’ve deleted it and replaced it but it keeps happening.

When this happens, the navigation menu changes from a one-line to a two-line menu because the last item wraps to the next line.

How do I get this last one to stay in place? Is there a limit on the number of items one can have in the navigation menu?