Currently, the reference are previewed in Jabref without the field month, as shown in the following:

However, the bib should be and is compiled correctly as follows (in which the month is displayed):

This is consistent as the ieee.csl fromcitationstyles.org

That is to say, jabref handles the field month correctly, but the preview with ieee.csl lack the display of field month.
I think this is an error and should be fixed.

Thanks guys for the great jobs.