A recent update to Microsoft Outlook that has been released has applied to really badly design display to make it look more like OWA.
This change has caused issues for many users with the autocomplete email address that appears when you type the email address in.
The list pops up instead of dropping down and cuts the list off with no scroll function.
Microsoft has recently released an update from testing on 01/07/21 which seems to now resolve this issue.
A temp workaround before this update was to revert back to an earlier version and use WSUS to prevent update reapplying
“C:\Program Files\Common Files\microsoft shared\ClickToRun\OfficeC2RClient.exe” /update user updatetoversion=16.0.13901.20462