Where a flex or grid container is smaller than the items inside, and has overflow auto or scroll set on it, items aligned/justified to end or flex-end behave differently to those aligned start or flex-start. The original post from an author who asked me the question is here. I did a couple of tests here, and found the same behaviour in grid with justify-content at least.

This seems to relate to this open Chrome issue, although the behaviour is the same in Firefox.

The relevant part of the spec is here, so the scrolling to the end of the container seems correct, but I think that most authors would expect the overflow to scroll, just from the other direction.

I'm not clear from the spec whether this is a bug in implementations, or is desired behaviour for a reason I don't fully understand, either way it could be clarified perhaps with an example (which I'd be happy to put together once I understand the issue fully myself!)