Created attachment 340569 [details]
Concise test case for reproducing the issue
Open the attached test case and move the mouse over the squares. In both cases the color should fade from red to blue.
In the left example, where color and fill="currentColor" are set on the same element, the transition is ignored and the color changes abruptly from red to blue.
In the right element the color is applied to the containing group element. The transition is applied correctly.
Still present in Safari Technology Preview Release 130 (Safari 15.0, WebKit 16622.214.171.124.5).
Bug is also present when `stroke: currentColor`
Both Firefox and Chrome have the correct behavior here, Safari is the outlier.