Hmm… then that appears to be the issue.
Myrddin should enter that thinking state after you have spoken to him at least once and either heard one of his jokes or fully explored six pieces of furniture.
Based on what you have described, you have completed more than enough for those conditions to have been met, which makes this particularly strange.
I believe I may have found a workaround for the issue, however.
Could you tell me which version of the game you are playing?
Are you playing through the browser, or using the downloadable version?
If you are using the downloadable version, please let me know whether you are on Windows, Linux, or macOS.