I originally posted this here: https://community.appinventor.mit.edu/t/tablearrangement-contents-fill-parent-issue/12708 but now I'm reporting it as a bug.
Place a TableArrangement on a screen, give it say 2 rows and 3 columns and set its width to Fill Parent.
Place a label in each of the TableArrangement cells and set the widths of any one column to 'Fill Parent', none of the labels expand.
I can manually set the label widths ok but they wont do a 'Fill Parent' and fill the table.