I’ve been running into the same issue as well, but was able to solve it
in my case i was using the grid panel for a grid inventory (which needed to display an exactly 3x5 grid)
create 1.0 fill rules for the desired size, for me it was 5 rows, 3 columns
at runtime before you populate your panel
create a new inventory widget that takes up one slot
then add this dummy widget to the panel
set the widgets position to the maximum that the panel should allow the [2,4] position
the widget is then removed and the grid can be populated with real entries like normal
this seems to be a bug with updating the fill rules at runtime, as the panel behaves almost like a uniform grid panel when enough fill rules are added and the last row and column have not yet been rescaled