Solution:

Step 1. Go to gear icon (settings). Under "TOOLS."

Step 2. Select "Manage categories." In the upper-right corner

Step 3. Toggle (switch on) "Personal categories."

Unfortunately, the "Personal categories" toggle (switch off) again when you run a report and then click on particular category (which links to transactions). No worries, simply switch on "Personal categories" again (Follow steps above.)

Rinse and repeat 15-20 times an hour.

It's tax season. Having fun yet?

I found and reported this bug with the QuickBooks Self Employed team at Intuit.