Scenario: I compose an e-mail on Outlook for Mac, and Outlook helpfully assists with keeping a numbered list going after I started typing 1. <stuff> <return> 2. <stuff>

I then click send. The e-mail runs through the Exchange server and comes back to my inbox, due to being part of a distribution group that I wrote the message to. However, the ordered list that spanned 1-19 comes back with two 10s, no 12 (I believe this is a crime under Seattle Seahawks fan laws), two 15s and no 16.

I open Outlook for Windows, and look at the same message in the inbox. It has the proper 1-19 sequence with no gaps and no duplicates. I then question my sanity, and check in Mail.App in macOS too. Same gaps and dupes as Outlook for Mac.

A person replies to the thread from Outlook from Windows. When I view it in Outlook for Mac or Mail.App my list in the quoted reply shows properly 1-19. Loss of sanity official now.

What is going on here beyond the typical old Microsoft corporate shenanigans that bleed out into their products?

I suspect the outlook client's hidden formatting can get twisted from time to time. Depends on the number of edits, cuts and pastes.

I think I've pinpointed your problem.

When in doubt, go have a look at the raw message, which will (hopefully) be in a tolerable mangle of HTML and see if it's properly structured as HTML. There may also be a plaintext alternative version in there worth peeking at as well.