nabla: Ah, so it is!  I'd searched for an existing bug report, but obviously not well enough.

Have just started playing at DailyGammon (as "wetware" there, and also at NetGammon).  Will check their multi-game .MAT output shortly, though I presume it's correct.  Since I now play enough bg matches here that are worth analyzing afterward, I might try creating a small Windows executable to auto-reformat BK's .MAT output (no matter how long a match), to make it compliant.