Given Mosaic Page layout:

+-----------------------+
| Contentlisting #1     |
+-----------------------+
| Filter Tile with      |
| content_selector of.  |
| Contentlisting #2.    |
+-----------------------+
| Contentlisting #2     |
+-----------------------+

What I expect:
Filter Tile has filteritems of Contentlisting #2

What you get:
Filter Tile has filteritems of Contentlisting #1

Why?
Because the ICollectionish adapter always sets self.collection of the first found contentlisting tile when initializing. See here https://github.com/collective/collective.collectionfilter/blob/master/src/collective/collectionfilter/tiles/__init__.py#L91

Solution:
self.collection should always be set to the actual found contentlisting tile in selectContent() here https://github.com/collective/collective.collectionfilter/blob/master/src/collective/collectionfilter/tiles/__init__.py#L102