5/28 of the sub-tests look to be vertically aligned slightly different (but are otherwise identical).
<rdar://problem/89219642>
It looks like this is an existing issue, that isn't specific to subgrid. https://webkit-search.igalia.com/webkit/rev/1cc94bc83c8db7dbc704f108d37fe29909a06736/Source/WebCore/rendering/RenderGrid.cpp#1404-1410 It looks like we give up trying to compute a baseline for the grid container if the selected child is orthogonal, which it is in this test.
Created attachment 456070 [details] Reduced test case.