Hey Ben,
first of all, thank you for the purchase!
It took me a while to figure it out, yet in the end it’s the absence of content that causes this effect. It is not a bug, rather the behaviour of the theme’s code given that every page in the one-page mode only has its title, i.e. has a very small height. This leads to the situation where the last three pages (or two, depending on the screen height of the viewing device) all fit into visible screen height at the same time. Since the main menu items are being highlighted based on the positions of their respective pages relative to the window, the menu considers that the highest page to still be visible corresponds to the current menu item, and thus the last menu item never gets highlighted as it never has the opportunity to be “alone” in the window :) The solution to this situation is quite simple: filling in the content into each page. As soon as they all have some “meat” on the bones, the menu behaviour should return to the expected norm.
Should you have additional questions, do not hesitate to ask!
Sincerely,
Andrii / Satori Studio