ProximityPrompts have a 1px gap when using the standard UI
To reproduce, insert a ProximityPrompt and hold the interaction key. Once the circle goes past 50%, you’ll notice the gap. This might depend on your screen resolution or camera angle, etc since it is likely a rounding error.
Image downscaling does not mean things become unaligned by 1px - this is almost always a rounding error caused by human error.
For example, Guis have AbsolutePosition which returns a decimal. You have to round the value to figure out which pixel it’s rendering on - but some people assume you can just drop the decimal, which leads to issues like the OP.