Here's a FWIW: after a bit more poking around, I've confirmed that MUAs Pine on Linux and The Bat! on MSWindows confuse a long series of dashes with the ^--boundary--$ string that is the end of the digest. I've submitted a bug report to the Pine folks at the University of Washington, and maybe someday they'll fix it. One of my digest readers has submitted a report to The Bat! folks.
In the meantime, I've added a workaround recipe to my digest's rc.local.s10 file that should prevent those buggy mailers from truncating my digests.
# #Quote long dashed lines, for broken MUAs # :0 B * ^-------------------------------+ { MID=`formail -x Message-ID` LOG="article $MID: dashes escaped for broken MUAs " :0 fwb | sed 's/^-------------------------------/- &/' }