Hi!

-- >8 --
.Dd
.Dt STTY 1
.Os
.Sh DESCRIPTION
.%B Seventh Edition Ns -style
-- >8 --
produces
-- >8 --
DDEESSCCRRIIPPTTIIOONN
     S_e_v_e_n_t_h_ E_d_i_t_i_o_n_-style
-- >8 --
on groff, but
-- >8 --
STTY(1)                     General Commands Manual                    STTY(1)

DDEESSCCRRIIPPTTIIOONN
     _S_e_v_e_n_t_h _E_d_i_t_i_o_n _N_s _-_s_t_y_l_e

Linux 5.10.0-17-amd64                                    Linux 5.10.0-17-amd64
-- >8 --
on mandoc (CVS HEAD and 1.14.5-1 off Debian).

mdoc(7), In-line says:
  Macro   Callable   Parsed   Arguments
  %B      No         No       >0
so while I think this accurately reflects mandoc behaviour,
that's incompatible with groff.

In /usr/share/groff/1.22.4/tmac/doc.tmac I see, simplified:
  .de %B
  ...
  .ie \n[doc-is-reference] \{\
  ...
  .\}
  .el \{\
  .nop \*[doc-Em-font]\c
  .doc-print-recursive
  .\}
  ..
and likewise for %T: they are Parsed.

Best,
наб