If you are using Mendix Studio (Web Modeler), it’s likely due to being upgraded to Mendix 8 Beta 3. You could start using Mendix Studio Pro to avoid further automatic upgrades.
If using Mendix Studio Pro, it must be due to an app change as you wouldn’t get auto-updates.
In either case, in order to fix this you’ll have to style the menu to look appropriate, unless Mendix classes this as a bug. You can raise a support ticket if you think this is a bug.
Hope this helps