It doesn't matter if there is a space in the text or not, it doesn't work either way. Yes, I would expect an empty string to fail, as I said this is just one small piece of code, I actually have checks in place to make sure the string isn't empty in the full code.
The style never changes no matter if it is a block or mtext.
I have attached a drawing with the original mleader in it, the ones with the blocks can't seem to be changed to having MTEXT and I am not sure why.