If you breed a butter/lesser bel to a normal, from everything I've heard so far, all of the offspring will be butter/lesser (not distinctly one or the other). Since you can't tell them apart, what you choose to label them as is up to you. Originally, butter and lesser looked distinct, but as continued breeding has been done, they're a lot less distinct now. Unlike Mojave/Lesser, there is nothing to suggest that butter and lesser aren't simply color-bred versions of the exact same mutation (lemon pastel and Graziani pastel would be another example of this).
Mojave and Mystic look extremely similar in some cases, but when you breed them, what you produce is different. A mystic potion does NOT look like super-mojave BEL. Therefore, it's obvious that mystic and mojave are different mutations. The same cannot be said of lesser and butter.