Numbered list adding tab spaces bug?

I am trying to make a numbered list but when the list gets to “10.” it starts adding what looks like tab spaces between the number and whatever text is behind it.

Is this a bug or some kind of setting somewhere?

This is not a bug - although we have tdf#55053 for it. See comment 22 there for the explanation. In essence: the spacing in the list is using a tab with some position that provides enough space for short numbers (1-9), but is too small for longer numbers; in that case, the tab jumps to the next position.

You may increase the list level’s tab position, or use space instead of tab (which would make the text after the number not aligned across items).