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This gets said a lot here but honestly I'm a bit skeptical. There certainly does seem to be a strata of society who, while not capitalists, are doing well enough under capitalism that their interests don't align with the proletariat at large. Perhaps there are more accurate terms, like "labor aristocrat" or whatever, but in American parlance, it does end up getting translated to middle class. And we see it in American politics, Trump's most active supporters fit almost perfectly into the American perception of what we call "upper middle class".

Also, I'm not the biggest Marx expert and I kind of suck at remembering sources, but I'm pretty sure both Marx and Engels have actually talked about this. Marx did use the term middle class on occasion, and Engels talked about how the workers of certain wealthy nations were "bourgeois proletariat" because they had access to market speculation through stocks and real estate.