This gap is called a ‘conflation artifact’ - the cause is that both the round rect and stroke are anti-aliased by adding some transparency to the color. If the shapes overlap in a certain way, the additive transparency still shows some of the background through.
In general, we can’t guarantee that all seemingly reasonable combinations of stroke + corner won’t cause conflation artifact. You’ll need to adjust sizes accordingly to work around the gap.
There are other anti-aliasing techniques roblox could use (like MSAA or supersampling) but there are other tradeoffs to make like efficiency/performance.