Hey Leo,
It's been a while ago now (almost four years), but I have found lots of "Big Seven" anion stock standard solutions from vendors that were "contaminated" with a preservative that eluted underneath fluoride...two vendors were kind and patient enough to supply lots of the solvent they used to make their standards. On those occasions, the "contaminants" were traced to less-than-ideal lots of preservatives from the vendors supplying the vendors that prepared the stock standard anion solutions we purchased...if that makes sense.
If you have some clean NaF, it may be worth the time to prep it and run with your own high resistivity water for your piece of mind. I've not run methanesulfonate routinely in my past, but the same may apply...if you have the time and the reagent(s) on hand.
Oh...and also, you can inject eluent...to review another potential cause.