There's a defect in 13.3, 14.2, 14.3, 14.4 and 15.1 that I've been advised is not going to be resolved.
Since I couldn't find any mention of this anywhere, here's the detail for the next person to stumble upon
CLRT-71520 When granting portlet view rights via group, favourites menu instances show.
As you can see in the screenshot, we have nearly 15,000 portlets
Whilst this is a minor inconvenience and I can still add the portlets to the group, it may cause some confusion to new administrators. It also slows the loading.
Steps to re-create:
I see the same in 13.3 with only 700 Favorites Menu portlets
Would depend on the number of users that have existed