I just discovered this bug as well. I thought it was only on the Mac app Preview Beta, but I confirmed it in the web search as well.
Searching did not return results on notes in a shared notebook, until I had opened the notebook. After that, the notes in the notebook appeared to be indexed.
I can't reproduce the bug easily but suspect that it may recur when there's a version update? The failed search was working in stable Evernote for Mac, but not working the first time I ran it in the Preview Beta or the web interface.