In the attached test case, the purple float is positioned outside the black border of the multi-column block. It should be inside the block, in the second column, right below the yellow rectangle.
Created attachment 66067 [details]
Floats just shouldn't be allowed to overhang into following sibling blocks.
Created attachment 66233 [details]
Created attachment 66234 [details]
Here's a patch with the layout test move.
Fixed in r66612.