LaTeX complains about overfull hbox but no box is actually overfull

In the following mwe I’m redefining the equation* environment to store the width of its contents in a token list:

documentclass{article}
usepackage{showframe}
usepackage[fleqn]{amsmath}

% from https://tex.stackexchange.com/a/59955/172923
makeatletter
newcommand{settowidthofalign}[2]{%
  setboxz@=vbox{
    begin{align*}
    #2
    ifmeasuring@elsegloballetgot@maxcolwdmaxcolumn@widthsfi
    end{align*}
  }%
  begingroup
  defor{+}edefx{endgroup#1=dimexprgot@maxcolwdrelax}x}
makeatother

ExplSyntaxOn

dim_new:N l_eqn_width_dim

cs_set_eq:cc { oldequation* }{ equation* }
cs_set_eq:cc { endoldequation* }{ endequation* }

RenewDocumentEnvironment{ equation* }{ +b }
  {
    settowidthofalign{ l_eqn_width_dim } { #1 }
    begin{oldequation*} #1 end{oldequation*}
  }{}

ExplSyntaxOff

begin{document}
begin{equation*}
  (b_{n + 1}, e_m) = (v_{n + 1}, e_m) - sum_{j = 1}^{n} (v_{n + 1}, e_j) ,
  (e_j, e_m) = (v_{n + 1}, e_m) - (v_{n + 1}, e_m)
end{equation*}
end{document}

however, when I run the code I get the warning

Overfull hbox (1.07913pt too wide) detected at line 37

even though the content of equation* is clearly within the page margins:

I suspect the warning is not generated by equation* itself, but by align*, which is used in the settowidthofalign macro. Indeed, commenting out

% settowidthofalign{ l_eqn_width_dim } { #1 }

gets rid of the warning. How can I fix this?

documentclass{article}
usepackage{showframe}
usepackage[fleqn]{amsmath}

begin{document}
begin{equation*}
  (b_{n + 1}, e_m) = (v_{n + 1}, e_m) - sum_{j = 1}^{n} (v_{n + 1}, e_j) ,
  (e_j, e_m) = (v_{n + 1}, e_m) - (v_{n + 1}, e_m)
end{equation*}


align
begin{align*}
  (b_{n + 1}, e_m) = (v_{n + 1}, e_m) - sum_{j = 1}^{n} (v_{n + 1}, e_j) ,
  (e_j, e_m) = (v_{n + 1}, e_m) - (v_{n + 1}, e_m)
end{align*}
end{document}