Haven’t seen that before. I can see why Bubble would do that. But I think it’s worth classifying it as a bug. (Ideally, Bubble would give the borders to all cells).
Regardless, I might recommend a different visual when it’s possible to have an “empty” space in a repeating group matrix. (Ie. have a border for the repeating group itself, but do not use a separator. Rather/or, group all of the contents in the cell in a group and apply a border around that).
There are a few “hacks” - like @vlad’s Bubble Hack: How to create a tile repeating group with an option to add a new item inclusion of an item on the end of a list…In this case, you would only apply it when the number of results is an odd number).
Dan (creator of LearnTo - 15+ hours of Bubble tutorials and live coaching)