Steps to reproduce the issue

1. Create a component view with two layouts, one default.
2. Create a menu item for each layout.
3. In the component router register the view.
4. Use JRoute::_ to render a route to the default layout.

Expected result

For JRoute::_ to return the URL for the menu item associated with the default layout.

Actual result

JRoute::_ returns the menu item for the non default layout.

System information (as much as possible)

Fresh install of Joomla 3.9.20

Additional comments

This is caused at lines 256 to 259 in \Joomla\CMS\Component\Router\Rules\MenuRules::buildLookup.

If a menu item id is set for is set for the default layout of a view, then its menu item id will be overwritten by any other layout from that view.