Any difference in behavior will probably be picked up on by our customers immediately.
Probably the best solution then would be to make sure that it doesn't look like a menu bar, but instead looks like a set of buttons.
This can be done with colors that are different than the background. A user would then expect a different behavior. I have been using this solution in a customer application for many years with 400+ users. The application has hundreds of child windows which are all based on a data-driven framework. Each child window has a menu across the top which are a set of buttons with pull-down menus (very similar to the sample).
There has never been a complaint that they cannot navigate horizontally when a menu is pulled down.