Looks like there's a gap on the display range. When it goes blank it means your fitness is between 80 and 90. You still have a value, the game just isn't showing it.
The only thing you need for Roxy is to be able to fight. If your fighting has a value of 10 or more you'll get a bonus to your relationship with her.
Your fighting increases by 2 when you visit the lodge, unless your fitness is at least a 60. Then it goes up by 4. A fitness of 60 is described as 'decent'. It might be possible to get all of Roxanne's events without combat skill if you butter her up enough through the correct dialogue choices, but the combat skill really, really helps.
Thanks for reporting that bug.