In "Dictionary of Bible Themes" punctuation has to be included if that is part of a Headword e.g.
headword:"baptism, in the Gospels".
Thanks for the report, I was able to repro and my search query needed to have the "," as you mentioned above.
Let me check with our team if there is a reason why and report back. If both queries should work e.g headword:"baptism, in the Gospels" AND headword:"baptism in the Gospels" -- I'll let you know and get a case written up.
Okay, did some investigating and this appears to be by design when using the headword keyword as it does require some punctuation.
It needs to be documented properly in Logos/Verbum Help