Could be due to 2 reasons:
- Recordtype on Global Actions.
- Override the Salesforce Mobile and
Lightning Experience Actions panel, in the Classic Pagelayout editor.
Firstly I created a recordtype on the task object.
This did not help, and was not required for me, but it helps for understanding.
2nd and more importantly, overrode the Salesforce Mobile and Lightning Experience Actions panel, and added the task/event global actions to the Classic Page layout.
This worked but only the New Event option appeared, not the New Task.
This was due to the new RecordType I had just created on Task.
So I had to edit the New Task Global Action to include the specific Recordtype for that action.