I grouped an image with its (manually-created) caption box, then set the group to wrap top and bottom. As I drag it from right to left, into the column, the column border blocks the caption text from entering the column. (See pointer in attached screenshot.). Notice there is no problem with the other, similar image on left facing page.
It looks like you may have in advertently enabled text wrap for the text column underneath it (column 3).
Oops! You're right! That was very odd to have done that! Thanks!