I guess this is a bug?
A member account can see (partially) the activity log of a folder that he’s not a member of.
I don’t have more examples but maybe it’s only some items or a particular attribute:
So the non-member can see the content of the long text attribute, but not the item and the folder name (they show as “Deleted Item” and “Deleted Folder” instead).
In contrast to that, this is what a member of the folder sees: