please make sure the sync of tasks works properly
during the last week there where times when i could not see 1 task in outlook for mac for hours after a reboot:
new tasks where synced but old tasks in the database could not be seen on mac even when they could be seen on ios.
But when i updated a single task on ios it would be seen synced on mac tool.
We found a problem with tasks is because the list of tasks is built by Spotlight - part of macOS. If the Spotlight index is not working or somehow corrupt, all your tasks wont display properly. You can rebuild the index just for the Outlook folder in /Users/username/Library/Containers/com.microsoft.Outlook I hope this helps someone while MS fixes it.
Jason Schadow commented
High Sierra and the Outlook December 2017 update (15.41.0 17120500) have this issue. The best workaround at the moment is to Empty Cache for the task folder... but missing tasks occur again at a later point.
Jeremy Cath commented
seeing similar issues in r15 ... add a new task and it doesn't even appear on the local task list for some minutes. Deletion also takes a while to update list and sync