Consider running attached HelloMenu code.
There's a single menu bar with the following structure:
Debug > Submenu > Submenu133 > Item 1
Debug > Submenu > Submenu133 > Item 2
Debug > Submenu > Submenu134 > Item 1
Debug > Submenu > Submenu134 > Item 2

Navigate so that Submenu remains opened all the time:
- go on Submenu133 -> you see Item 1 & Item 2, fine (attached capture Submenu133FirstAccess)
- go on Submenu134 -> you see Item 1 & Item 2, fine
- move back on Submenu133 -> uh oh, Item 1 and Item 2 are no longer visible (attached capture Submenu133NextAccess)
- move again on Submenu134 -> it is empty too

If you completely close Debug menu or simply move over Debug > Open and starts again to crawl in Submenuxxx you reproduce all the steps: basically second level submenu's content becomes blank the second time you access it in the same navigation.
If you remove Submenu134 to keep only Submenu133 then all goes fine.