Since bold text is set to be displayed by overlapping identcal gryph shifted to right by 1 pixel which is based on Mojangle(the default font), custom fonts which is thinner than Mojangle displayed as doubled when they are set to bold with gap between original gryph and overlapping gryph.

Similar bug have been reported as MC-76356, and it was resolved by changing the amount of shift to 0.5 Mojangle pixel, and there should be field like "bold_shift" to specify how much shift apply to overlapping gryph on ttf gryph provider to resolve this issue.