While checking some sets for the behavior of Visual Bug in Visual Spoiler if stumbled upon another:

This visual spoiler page clearly shows frames of multiple colors, but for me the line "Show frames:" contines only a button for filtering black frames (the set prviously contained only a large number of black cards, so maybe the buttons just don't update correctly with new cards added?).

Force refresh does not resolve the issue.

Huh. There was something very weird going on there.

The first thing I tried did fix it for that set, but sadly that meant I wasn't able to investigate why it had happened. It's probably some quirk of cacheing that could still happen in future, so I'll leave this open.