This makes some sense, although it may not be ideal.  The "leading space" is really an RFC 2822 continuation line, so that whitespace at the start of the second line is folding whitespace.  Technically those two lines should (and do) get collapsed.

The question is whether this message should be printed idempotently.  In general the email package tries hard to maintain this principle, so I think it's probably a valid bug.  However, it doesn't affect the semantics of the message so it's not a high priority fix.