Wrong line numbers on swapping of macro arguments

[matze-dd]

Assume declaration

Macro(name='swap', args='AA', repl=r'\2\1')

Then the input

\swap
{A}
{B}

correctly leads to output 'BA', but the corresponding line number from option --nums is '3+' instead of '2+'.

This only happens on swapping of arguments, and it apparently has been introduced with the simplification of mysub() in release 1.5.6.