A few months in, we are hearing surprise from some users checking their "Junk Email" folder to find a proportion of the content clearly not Junk.
Microsoft defines Junk as "...spam, which are unsolicited and universally unwanted messages (when identified correctly)".
Office 365 servers use Microsoft's "Exchange Online Protection (EOP)" to spare us from phishing, spam and other inappropriate messages. In addition, machine learning is used to further filter messages that may be benign but unwanted.
It appears that the move to a new communications service has triggered some re-learning.
What to do? Keep a closer eye on your Junk Email folder in Outlook 365, at least for the next few months. When a non-Junk message is found, either right-click and select "Mark as Not Junk" or use a different right-click command to add the sender to one's contacts. Outlook should not flag mail from contacts as junk (unless discovered to be malicious). Outlook will (re)learn and the false-positive rate should go down.
Junk Email settings are configured by AHS. It is not possible for users to opt-out through user preferences. This makes sense because EOP prevents hacking (especially phishing) as well as spam.