I’m afraid I didn’t do anything special.
I was logged in as admin to update, then impersonated a test user. As the test user, I posted a message to a category with Approve Posts set, and then went to look for it in Activity/Pending.
I’ve refreshed, and logged out and back in as the test user (rather than impersonate) and it’s still blank.
I’m using Android, Chrome.
Update:
It’s the same on my live site as well.
And it does the same on my desktop (Windows10, Firefox):
Have I misunderstood something? (it happens fairly often 
)