Gotta admit, I had a similar problem with my negs when I used Pan f as well with flash.
My case I used Bowens 500's in softboxes and metered for the flash as I have a thousand times before. But devving it they came out just like the OP. When I've devved using with natural light, its one of my favourite films. I gotta admit after using the 3 rolls I did at the shoot in the studio I've been turned off using Pan F in a studio setting.
I should add that I shot digital alongside the Pan F and that was perfectly exposed, I used that method as I wanted to be sure my settings were right and not just blindly following the meter.