Buttons inside of a button matrix are always aligned to top left corner

Lets take the following example code
https://docs.lvgl.io/8.3/widgets/core/btnmatrix.html#pagination

If I change the border width of the main container to greater than 0 here >> lv_style_set_border_width(&style_bg, 0);, there is always a gap towards the right and bottom side between the buttons and the border. This does not appear to happen while trying the micropython simulator online. I have tried tweaking padding, shadows and alignment, but no luck. Can someone please help.