Nope, that's not it actually, all punctuation works.
I didn't realise that this is the only file where had eqn delims on.
Are you ready?

  .de St-p1003.1-2024
  .Tn \%IEEE No Std 1003.1-2024 Pq Dq Tn \%POSIX Ns .1 \\$@
  ..
  .EQ
  delim @@
  .EN
  .St-p1003.1-2024
produces
  ERROR: incomplete escape sequence: \$\&
  ERROR: argument number is not numeric: \$\&
(and the other one on older mandoc).

Because rather than parsing \$@ correctly as "all arguments",
eqn somehow sees this (I don't even think it should under normal troff?)
and tries consuming @...@, and replaces it with the rendered output of \&.
Thus, I think, \$\&?