I found a solution so I want to share it here if someone would have similar problem.

If you are using a thermal receipt B&W printer, make sure everything what you're trying to print has 100% black color. So in this case, the problem was, that texts were not black, but just some kind of dark gray which resulted in white dots on the print.

The main reason of bad color was bootstrap, so the solution is forcing the text to be black by adding color: black !important in stylesheet.