Okay, I have finally fixed this. The issue was that when I installed the Sales Navigator plugin, I hadn't already had a Task type, so it defaulted to the LinkedIn task. I was able to fix it by following the instructions under "KEY CALLOUTS" here:
Note: Installation of Sales Navigator will create new Record Types for Tasks and assign custom defaults. If you do not have “Task” Record Types created, this can cause Record Types for Global Actions (e.g. New Event, Log a Call, New Task, etc.) to default to "--Master--" which would remove existing Global Actions from the Activities list.
If after enablement you find that Global Actions are missing, please take the following steps:
To Start this process, navigate to Objects and Fields in the Setup menu using “Quick Find”.
Next, click Object Manager followed by Task. From here, click on Record Types followed by “New” in top right corner.
Create a new Record Type labeled “Generic Task” or desired custom name. Make it Active
and Enable for desired profiles.
Next, from the Setup menu “Quick Find”, navigate to User Interface and then click Global
Actions.
Click “Edit” for each of the missing Global Actions, then click on the Record Type drop down
menu. Select the custom Generic Task you previously created and then Save
Task.Status
field