...only if it is the first token ...macro name...the second token....should...

Macroinstruction defined using the struc directive must be preceded by a label (like the data definition directive) when it's used.

hopcode,

The only source of such information is the FASM source.

(...)It begins with checking for the preprocessor directives, and when none of them is detected, preprocessor checks whether the first symbol is macroinstruction. If no macroinstruction is found, it moves to the second symbol of line, and again begins with checking for directives, which in this case is only the equ directive, as this is the only one that occurs as the second symbol in line. If there's no directive, the second symbol is checked for the case of structure macroinstruction and when none of those checks gives the positive result, the symbolic constants are replaced with their values and such line is passed to the assembler.

No, I haven't. There is additional code that recognizes the case when first token is not a macro name and second token is ':'.

There is one more special case - when preprocessor goes to checking the second symbol in the line and it happens to be the colon character (what is then interpreted by assembler as definition of a label), it stops in this place and finishes the preprocessing of the first symbol (so if it's the symbolic constant it gets unrolled) and if it still appears to be the label, it performs the standard preprocessing starting from the place after the label. This allows to place preprocessor directives and macroinstructions after the labels, analogously to the instructions and directives processed by assembler, (...)

macro a arg {
  local done
  define done -
  match =:, arg \{
    a:
    define done +
  \}
  match =-=:v,done arg \{
    b: v
    define done +
  \}
  match =-, done \{
    db `arg
  \}
}
a: db 'A'
   a  B
a: