So if she's doing both at the same time, then by definition DW can't be her full-time job. I know people have suggested that she might only have a small part in it but she's playing the main character's sister, which is actually one of the main supporting roles.
Michael J. Fox famously filmed
Family Ties during the week and
Back to the Future (the first one) on the weekends. They wouldn't downsize his role on the sitcom to afford him time to film the movie, and they structured his shoot on
Back to the Future in such a way to maximize his two days per week on set.
I am not saying that Jenna is as important to this film as Fox was to
BTTF, but there's nothing particularly weird about this situation, nor does it mean that Jenna is no longer full-time with
Who. If the producers cast Jenna knowing that she had commitments for
Doctor Who, then they almost certainly set up the filming schedule to accomodate that.