AFAIU, a line starting 'From ' *is the message header*. If such a
line occurs in the body of a message, it should be stored as '>From '
(i.e. with a '>' prepended). This seemed to complicated to some folks
so they wrote more "clever" mail programs that looked at the whole
line to determine if it looked like a real message header and if not
left the 'From ' unquoted. Unfortunately, this produces mailbox files
that are almost but not completely compatible with the "quote all
'From 's" style. You are seeing the results of this incompatibility.
In short, this isn't a Yarn bug, but rather yarn using an different
standard for the unix mailbox format than your other mail client.