Baja,
Some others dismissed the notion that a fractional might demand you resign seniority but obviously you have discovered that is the case with at least one fractional. And who can blame them? If you were an employer, would you want to invest thousands in a person that stood a better than average chance of departing the pattern the minute their major turned the corner?
Obviously, not all fractionals make that demand and good for them. A good fractional management's attitude might be to try and make the pay, benefits, and QOL so good that their employees chuckle at their former co-workers at the major beating their brains out for less money, poor job protection, lousy retirement plans, and terrible schedules.
As I posted before, I would still apply to every fractional, take every interview offered, and then pick the best spot for your situation. Even though I think you will prefer fractional flying to the Mess in Minneapolis, DON'T resign the number.
Keep us posted. Many would like to know the timeline of application to interview, the interview itself, aircraft and domicile offered, etc.