I've spent thousands of hours using Opus and have never seen this. I'd double-check your claude.md files.

I've seen exactly this behavior on claude.com with no system prompt with Opus 4.8 specifically, especially around chronic illness stuff where there's established mainstream medicine dogma and reddit / internet communities with alternate causality theories and treatment approaches (PMDD and MCAS-adjacent illness). 4.6 is happy to analyze and consider them, 4.8 really doesn't like the alternate theories and treatments.

That's vanilla claude.com, without memory or custom prompt.

I use another service for coding.

It's interesting how my experience there is mirrored by the answers here, though!!!