Apple Mail - Incoming email on Mac from specific sender has no content but same email on iPhone is ok

Check the new privacy settings in Mail -> Settings -> Privacy. Uncheck Protect Mail Activity, Hide IP Address and Block All Remote Content.

These new privacy settings can most certainly result in emails appearing to have no body content whatsoever. This is not Apple's fault this is the sender who is rendering the entire email running external code. They are also very likely tracking you and invading your privacy in ways you are not aware.

I was informed that Apple is aware of an issue with mail where many emails may show "no content". Not sure when they will fix this issue but you can read this and see the possible (not easy) fix:

https://discussions.apple.com/thread/254504347

I would need to see the email raw source.

View menu -> Message -> Raw Source

or

Press ⌥ + ⌘ + U [while the email in question is selected in Mail]

You might not wish to share that publicly. So if you do post it in reply, please redact any personal private information. I don't need to see your email address or any other personal details. But I do need to see how the email is constructed and formatted.

No privacy setting did nothing for me. Only reinstalling the OS fixed the issue here.