Using the dev version from today.

JabRef accept keywords in the keywords field without (or with space) separator.
By default biblatex use comma (,) as separator. Not using it causing some problems e.g. when you filter your bibliography by keywords.

Separators in biblatex can be modified. So I think it would not be the best solution for JabRef to check for comma only.

There should be a warning or something like that if you have multiple keywords but no comma between them.