Never seen this before. In a few projects I have noticed that where I have say an image and text in a 2 col stack and then put text below in a single column stack or not in a column stack (i.e. just NOT in the 2 col stack), the text below continues in column 2 instead of being full/site width. Bug in Stacks 4? Or maybe incompatibility with one of the stacks? No idea but ideas welcome. If I put a divider-line stack below the 2 col stack the problem goes away.
Here are the stacks in edit mode. The ‘By Train’ header and text should be full site width.
However as can be seen in the image below, the text just continues from the right column above: