See: https://wpt.fyi/results/css/css-values?label=master&label=experimental&product=chrome&product=firefox&product=safari&product=webkitgtk&q=%2Fcss%2Fcss-values%2Fviewport-units-keyframes.html+or+%2Fcss%2Fcss-values%2Fviewport-units-invalidation.html+or+%2Fcss%2Fcss-values%2Fviewport-units-compute.html I think these are all going to be one bug, though it's kinda notable none of these occur on WebKitGTK.
<rdar://problem/91931206>
No longer seems to repro on STP 148; I wonder if somehow bug 234373 fixed this?