See this thread in the Logos 5 forum Hebrew morphology search with part of speech not working
Genesis 1:1 right click on beginning in the ESV. Select Lemma Morph Search with all parameters

Received 0 search results:

Remove any of the search parameters—removing the F(eminine) will give results that should have resulted from the original search.

Dave Hooton responded to the original thread that this did not occur in Windows. I believe he is using the 5.1b Beta. LimJK reported correct results using 5.1a SR-1 Mac. Therefore, it appears that this is a bug introduced in 5.1b Beta 1 Mac.
Can anyone confirm this?