I've messed with Mail using IMAP only accounts, and it behaves normally that I can tell.
It does mean that whatever cause of your experience doesn't seem like a widespread one. It would be curious to know what kind of content is missing, and if that's consistent. In the days of text/rtf based message content basically a simple client/server connection was sufficient to get mail reliably, but todays code heavy messages, frequently where the message is basically a graphics image, even 'blocking all remote content' in Mail privacy can get in the way. I do note you have those protections set to 'off' of course, so clearly it isn't that.
If you have web access to the mail, can you see content there that is missing in the Mail client? And perhaps it is worth testing with a third-party app such as Edison, just to try and narrow down the problem?
Before I ran the upgrade, I was on Sonoma 14.3x. I saw all the bug reports with 14.4.0 and held off.
This morning, the emails in question were displayed fine with the content. Afterward, the content didn't show. It's hit or miss, so I don't know what's going on.
In webmail, all content appears just fine (same Mac, using Safari). Same with my iPhone.
Examples of senders include LinkedIn, Starbucks, Pinterest, my church's weekly update, and others.
In some cases, one email from the same sender shows content, and an email from another date doesn't.
For now, I'm fine but may try a different app like Edison.
