DevTest has always has a maximum length for randomly-generated parameters. In really early versions, it was approximating max int + padding, then it moved to approximating to max long.
If you want longer than 36 random characters, I suppose you could create multiple string properties (longRand1, longRand2, longRand3) and concatenate them: longRand = longRand1.concat(longRand2).concat(longRand3).