I tested all recent Opus versions and 4.6 and 4.7 were both fine FWIW. Seems 4.8 is when something started to go wrong.

That seems about right. 4.8 is like in between 4.6 and 5 in terms of capability and language and they are all pretty close honestly. I just default to using Opus 5 for a coding agent that I don't interact with and I like driving with Opus 4.6 or Fable. Fable thinks too much though. Fable is like that engineer on your team that will over-engineer the shit out of something if you let them. Fable was like, "Here are 34 yaks, which shall we shave first <hands rubbing together>" and I was like can't we just... write the script first and then decide of any of these poor yaks need shaving?