I have almost 1300 subscribers. I spoke with the host and there’s no evidence of an issue.
Yesterday I deleted the message which was going out and I sent a brief email message with a link to the newsletter and it went out perfectly with no issues and I see the clicks and opens are normal level.
Given that, it appears that the issue is with the longer newsletters so I will have to test it again towards the end of the month and see what happens.
The workaround is fine for my needs because I have a physical file to link to.
Hopefully it’s a temporary thing which is resolved with the next update.