menu custom links on same domain being rewritten

Multisite installation. No active plugins on the instance.
The multisite instance is in a folder, like domain.com/subsite/.

I create a menu and add a custom link to that menu, linking to a page that is in the multisite base instance, say the href is domain.com/page/.

When I put that menu into my theme, the menu item link is now domain.com/subsite/page/.

No idea how this could be happening. It’s very frustrating.