Hi!

-- >8 --
.Dd
.Dt STTY 1
.Os
.Sh DESCRIPTION
.TS
c s
l l .
\_
b	c
.TE
-- >8 --
(reduced from a real table, as in I did actually run into this)
is rendered as such by mandoc 1.14.5-1 off Debian and CVS HEAD:
-- >8 --
STTY(1)                     General Commands Manual                    STTY(1)

DESCRIPTION
     ─
     b   c

Debian                         October 17, 2022                         Debian
-- >8 --
and as such if you 9s/b/bBbBbB/:
-- >8 --
STTY(1)                     General Commands Manual                    STTY(1)

DESCRIPTION
     ──────
     bBbBbB   c

Debian                         October 17, 2022                         Debian
-- >8 --

This is wrong: the cell that contains the line spans over both b and c.
Compare groff (1.22.4-6) with the correct render:
-- >8 --
DESCRIPTION
     ──────
     b   c
-- >8 --
and
-- >8 --
DESCRIPTION
     ───────────
     bBbBbB   c
-- >8 --

Best,
наб