Ehm, hello,

forget about this, this is an issue with mawk(1) which doesn't
properly (imho) fflush("") before END{}, resulting in partial
output which then irritates mandoc(1) (different to groff(1) which
passed over the "EOF" already in it's sequential processing):

  ?0[]$ awk -v TOC=Ss -f mdocmx.awk < mdocmx.7|mandoc|v
  ...
  TABLE OF CONTENTS
       SYNOPSIS

       DESCRIPTION

             Creating a table of contents
  ...

  ?0[]$ mawk -v TOC=Ss -f mdocmx.awk < mdocmx.7|mandoc|v
  ...
  TABLE OF CONTENTS
       DESCRIPTION
             Creating a table of contents
  ...

I've sent a mail to Thomas Dickey regarding this.
Sorry for the noise.

--steffen