menu incomplete on mobile phones with landscape format

Hi together

I manage a website with 8 menu items in the top menu. If I look at the page on a mobile phone in landscape format not all menu items are visible. (maximum 6 items) If I want to scroll down, the page is scrolling in the background and not the menu.
It doesn’t matter if the 6 items are in top menu or in submenu. There are always just 6 items shown.
If I rotate my phone to portrait format the full menu is shown, because it has enough space to show all menu items.

Did someone else has this issue and knows what to do?

Cheers
Michael