Why does raggedright not work inside a minipage in this case?

In trying to suppress underfull box warnings in annotations to equations in align environments, I’m experimenting with raggedright. I’ve discovered that it works inside a parbox but not inside a minipage environment. Why is that?

% !TEX TS-program = lualatexmk
% !TEX encoding = UTF-8 Unicode

documentclass{article}
usepackage{amsmath}

newcommand*{reason}[1]{&&begin{minipage}{3cm}{raggedrightsmall{#1}}end{minipage}}
newcommand*{thereason}[1]{&&parbox[c]{3cm}{raggedrightsmall{#1}}}

begin{document}
begin{align}
  begin{split}
    a &= b + c + d + e + f + g + h + i + j + k + l   
      &quad + m + n + o + o + q + r + s + t + u + v 
      &quad + w + x + y + z 
  end{split} reason{a really stupidly long equation split over multiple lines} 
  begin{split}
    a &= b + c + d + e + f + g + h + i + j + k + l   
      &quad + m + n + o + o + q + r + s + t + u + v 
      &quad + w + x + y + z 
  end{split} thereason{a really stupidly long equation split over multiple lines} 
end{align}
end{document}