Hi!

Reproducer:
-- >8 --
.Dd
.Dt STTY 1
.Os
.
.Sh DESCRIPTION
.Xr intr II
(\c
.%B "UNIX Programmer's Manual" ,
p. 122)
-- >8 --

groff yields:
-- >8 --
DDEESSCCRRIIPPTTIIOONN
     intr(II) (_U_N_I_X _P_r_o_g_r_a_m_m_e_r_'_s _M_a_n_u_a_l, p. 122)
-- >8 --

mandoc (current CVS, end-of-May CVS, Debian 1.14.5-1) yields:
-- >8 --
DDEESSCCRRIIPPTTIIOONN
     intr(II) (_U_N_I_X _P_r_o_g_r_a_m_m_e_r_'_s _M_a_n_u_a_l_, p. 122)
-- >8 --
and the same happens in -Thtml:
-- >8 --
<h2 class="Sh" id="DESCRIPTION"><a class="permalink" href="#DESCRIPTION">DESCRIPTION</a></h2>
<p class="Pp"><a class="Xr" aria-label="intr, section II">intr(II)</a>
    (<i class="RsB">UNIX Programmer's Manual,</i> p. 122)</p>
-- >8 --

The same happens if the ""s wrapping the title are removed.

Oddly, the punctuation is correctly welded to the text,
but the font sticks?

A perusal of the TODO shows this item:
-- >8 --
- .%T can have trailing punctuation.  Currently, it puts the trailing
  punctuation into a trailing MDOC_TEXT element inside its own scope.
  That element should rather be outside its scope, such that the
  punctuation does not get underlines.  This is not trivial to
  implement because .%T then needs some features of in_line_eoln() -
  slurp all arguments into one single text element - and one feature
  of in_line() - put trailing punctuation out of scope.
  Found in mount_nfs(8) and exports(5), search for "Appendix".
  loc **  exist **  algo ***  size *  imp **
-- >8 --
which appears related, but for %T, not %B.

Best,
наб