Messages change when I inc and then packf

[kat]

Dear Colleagues,

Some of my emails have a curious property.

Consider the following procedure.

1. I start with an mbox, such as a maildrop.
2. I inc to a folder.
3. I packf the folder, producing an mbox containing the same messages
   as the mbox in step 1.
4. I inc the mbox I just created, producing a folder containing the same
   messages as the folder in step 2.

That is, I run something like this.

  inc -notruncate -file mbox-1 +folder-1
  packf -file mbox-2 +folder-1
  inc -notruncate -file mbox-2 +folder-2

With most of my emails, the messages remain identical in all steps.
But with some of my emails, the mboxes differ, and the folders differ.

The message in the starting mbox (step 1) has the return-path in the
first line,

  ------=_Part_5288516_462883164.1583074872825--

  From bounces+02f28c80-742a-11ea-bdd3-525400f0258a_VS2CRM@sender2.zohocrm.com 
Thu Jul 23 18:14:07 2020
  Return-Path:
    <bounces+02f28c80-742a-11ea-bdd3-525400f0258a_VS2CRM@sender2.zohocrm.com>

while the message in the ending mbox (step 3) has that token removed.

  ------=_Part_5288516_462883164.1583074872825--

  From   Fri Jul 24 07:52:09 2020
    <bounces+02f28c80-742a-11ea-bdd3-525400f0258a_VS2CRM@sender2.zohocrm.com>

The message in the starting folder (step 2) has a first line of
"Return-Path:",

  1 Return-Path:
  2   <bounces+02f28c80-742a-11ea-bdd3-525400f0258a_VS2CRM@sender2.zohocrm.com>

while the message in the ending folder (step 4) has a first line of "".

  1
  2   <bounces+02f28c80-742a-11ea-bdd3-525400f0258a_VS2CRM@sender2.zohocrm.com>

This is the only difference.

These emails seem to come from particular sources, including Google
Groups, Google Calendar, Topicbox, Amazon Simple Email Service, and
Zoho. For the unfamiliar, these are services as software substitutes.

First, are the emails formatted properly? If they are not, I don't care
so much about the situation. But if they are, what do I do to make them
stay the same when I inc and packf them?

Please accept the expression of my distinguished sentiments.
Krullen Van De Trap