Mac OS Mail messages go blank when moved to "On my Mac"

Hello, I am using Mac Mail on my Macbook Air. When I move my emails from "inbox" to my "read" folder which I have "on my mac" the content of the messages disappears. I have 4 email accounts and it only happens for one of the four. It happens for roughly 1 of every 2 messages and it started suddenly, 3 days ago. When I move the message back to "inbox" the content stays disappeared.

Based on suggestions in another thread, I have gone to Mail > Settings > Privacy and unchecked everything there. That didn't help.

The Mac is an M2 mac running OS Ventura 13.4.

Some possible factors:

(1) I traveled from the US to Germany 3 days ago. The problem started after I got here and started working from a local office using their network. Nobody else in the office has this problem, (nobody else here is using Microsoft 365) and it's limited to 1 of my 4 email accounts.

(2) The email account in question is being hosted on Microsoft 365; I do have another email account on Microsoft 365 and that one, though it has maybe 1/10th the traffic, doesn't seem to have the problem.

(3) Microsoft support says it's an Apple mail problem, and the solution, according to them, is to switch to Outlook, which I don't want to do.

If you have any ideas how I can fix this, it would be great.