@Klaas: Sorry to hear, have tested this with my vault (default light theme, many plugins, several CSS snippets installed) and the help vault, see screenshot.
I simply used the “Obsidian” & “Credits” notes from the help vault for embedding here.
CSS can be rather fragile, especially when a software changes, different themes and/or CSS snippets are used, or different options selected that influence how the parser builds HTML.
I’d suggest you try to test with the help vault first (its just 2 files: the snippet & a minimal note, plus one setting). If that works, try your vault using the default theme, no community plugins and no CSS snippets first, and work up from there.
Maybe it’s just a theme, plugin or snippet you use that makes the selectors in my snippet fail. The snippet isn’t long, after all, so I’m sure it can be made to work with your vault.
EDIT:
N.B.: I forgot to ask the most simple question: Did you add cssclass: clean-embeds to the note(s) you try this with?
“Clean embeds” was meant not as a “change-them-all” snippet but for single “master documents” only where you want a different embed style, so it requires the cssclass. It will not change anything on “normal” notes.