https://danross.co/flow/ In Safari 16.4 or STP 170 hover over any of the buttons. The "Info" and "Get Flow" buttons show the bug most obviously. The bottom left corner of the simulated drop shadow renders the edge incorrectly during the hover animation. It does not happen in Chrome, and I can make it happen, but much more subtly if at all, in Firefox.
Created attachment 466444 [details] recording of the bug This is very subtle but indeed it happens. There's a quick black thing which appears briefly below the box.
<rdar://problem/109645713>
This looks like a rendering issue, possibly clipping. Unlikely to be actually about animations per se.