The Master compilation of Ullage and propellant study
The research above shows that Argon is miscible in Liquid Methane at .49 But I believed the mixed liquid Argon with Liquid Methane, So Argon as a ullage gas for liquid Methane might be a lower process that also might form ices(?)
The master ullage gas study done by a PHd student
Some references to self pressurized LH2 and LCH4 propellants which is what ULA and SpaceX are working on, No references to noble gasses being used for good reason helium is light.
So its still an unknown as to how Noble ullages gas would react to chemical propellant