Fontsize not affecting some bbm symbols

The bbm package uses a font that's only available as bitmap. The related .fd file contains

\DeclareFontShape{U}{bbm}{m}{n}
   {  <5> <6> <7> <8> <9> <10> <12> gen * bbm
      <10.95> bbm10%
      <14.4>  bbm12%
      <17.28><20.74><24.88> bbm17}{}

and other similar parts. This means that the font is not available at sizes beyond 25pt (24.88pt, to be precise). Indeed, you get the warnings

LaTeX Font Warning: Font shape `U/bbm/m/n' in size <50> not available
(Font)              size <24.88> substituted on input line 6.

LaTeX Font Warning: Font shape `U/bbm/m/n' in size <34.99985> not available
(Font)              size <24.88> substituted on input line 6.

LaTeX Font Warning: Size substitutions with differences
(Font)              up to 25.12pt have occurred.

Solutions? Use a different font that's fully scalable, for instance STIX.

\documentclass[border=10]{standalone}
\usepackage{lmodern,amsmath,amssymb,bm,physics,nicefrac,fixcmex}

\DeclareFontEncoding{LS1}{}{}
\DeclareFontSubstitution{LS1}{stix}{m}{n}

\newcommand{\bbone}{\text{\usefont{LS1}{stixbb}{m}{n}1}}

\begin{document}
{\fontsize{50}{62.5}{\rmfamily 
$\displaystyle\int_\mathbb{R}^{} \sqrt{\dfrac{1}{\sqrt{2\pi\sigma}} 
\exp(-\dfrac{x^2}{2\sigma^2})}\bbone_{\{x>0\}} \dd{x} = 1 $}}
\end{document}