There is a bug in the current Office update, April 29, 2021 Version 2104 (Build 13929.20296). Microsoft is aware of the issue. The autocomplete menu is larger than in the previous build, lacks the ability to delete entries from the list, and may open upward.
This autocomplete list looked this like in the Outlook 2019 or 2016 (perpetual license), or in Outlook 365 with the DisableAutoCompleteUpdate registry value set.
What I know at this point: if you are using the DisableAutoCompleteUpdate registry key to use the old, simple autocomplete list, the autocomplete menu will open upwards. This key value is user-added, so if you didn't add it, it won't be in the registry at HKEY_CURRENT_USER\Software\Microsoft\Office\16.0\Outlook\Options.
If you have the key in the registry, delete or rename the key and it will fix the upward menu.
Deleting the key will not fix the inability to delete entries on the autocomplete list. We need to wait for Microsoft to fix that.
If you want to rollback your Office install, you need to set Office to display automatic updates (in any Office app, go to File > Account or Office Account in Outlook).
Type cmd on the Start screen to find the Command Prompt then right click on it and choose Run as Administrator
This command will revert to April 23, 2021 Version 2103 (Build 13901.20462)
Paste this into the command window and press enter.
"C:\Program Files\Common Files\microsoft shared\ClickToRun\officec2rclient.exe" /update user updatetoversion=16.0.13901.20462
If you prefer to go back one more build, the April 13, 2021 Version 2103 is 16.0.13901.20400