I was directed here after posting this question at Microsoft Answers: https://answers.microsoft.com/en-us/msoffice/forum/all/word-new-viewing-reviewing-behavior-with-version/ac53becb-f1b8-4721-b321-f324b8886a32
Word has a new behavior starting with Version 2308, a change from 2302: When Word launches with the document, the Viewing Reviewing button is now there, defaulting to Viewing, forcing the user to click Allow Editing in order to edit the document. This extra click will be disruptive to our users (about 200) when this version gets pushed out to them in the coming months.

The details: All the users are on our Intranet and have Word installed with our Add-In. The site is an ASP.NET MVC Solution we build, maintain, and host in-house.
- Users start on a page like [https]our[dot]site/Draft
On that page, they click a button to open their document in Word
Word, in turn, sends a GET back to our server requesting that URL and one of our servers responds with the Word Document generated on the fly.
- Before 2308, Word allowed immediate editing. Now the new Button "Viewing" appears.
Our organization has the Word Versions locked down, so we cannot get the latest versions. We can obtain Office only from
Semi-Annual Enterprise Channel
Semi-Annual Enterprise Channel (Preview)
Our user on Preview got 2308 and discovered the error that way. Our normal users will get that version soon via the standard "Semi-Annual Enterprise Channel."
We tested four different ways with the same binary Word Document
Via our App vs. opening as a local file and
With our Add-In Enabed vs. Disabled.
The summary is that with or without the add-in, our document is in “Viewing” mode when opened from the App. When opened directly as a local file again, with or without the add-in, the same binary document opens as desired in “Editing” mode.
So the problem is with Word opening a link on our website using the format ‘ms-word:ofe|u|<https link>’
Two questions: First, what was the change causing this new behavior? The Release Notes are not clear about this. Second, we need to know how to change this. We could change a setting on the user's Word Install if required. We could change how we construct the Word Document so Editing is the default since we have complete control over building the Word Document (done on the fly when the user clicks a link), so if there is a setting within, we should be able to set it appropriately. Finally, we are running an Add-In, so if there is some action the Add-In should perform, we could also solve it that way.
Many Thanks, John B.