Using the New Outlook for Mac features in business environments
Silly me, I forgot about the issues with the new Outlook, installed it, and found the same problem with lines. Then remembered I have to reinstall all my email accounts in Legacy.
Seemingly, nobody actually uses Outlook for Mac at Microsoft or they would have quickly run into this issue. I have gone back to Apple Mail which I dislike for other reasons.
It reminds me why I keep a Windows machine running on Legacy. This together with not being able to use Pop3 in Outlook on iPhone which has never been properly sorted.