I am using the Panel fields row style in some Views displays, and after upgrading to 3.9 I'm seeing empty fields in these displays. The rows themselves are in fact displayed, but they have no field content when output to the page in question. This is also true in the live preview when editing the View. Switching to another row style remedies this problem.
The views in question all use 1 argument, some filters, sort options and a couple of relationships.
I have compared the generated SQL from 3.8 to 3.9 and cannot detect a difference, but I guess that is expected behaviour. Running the query by hand does in fact provide a node_title column containing the title of the nodes, but it is not output to Views (when using Panels 3.9).
Reverting to Panels 3.8 correctly displays the Views that use Panel fields.
To be clear: I'm using the Views UI, but this bug pertains specifically to Panel fields which are maintined in the Panels module.