Have you verified that the MDM command is not already in flight for the update before unchecking?  

I have issued cancel pending and failed commands to the group several days ago.
 

That’s a chicken/egg thing - if it was already in flight and received by the device, then it’ll process the command even if it’s in a pending state

The iPads are powered off (and have been for months).

When they are powered on, the apps update.

JSS 11.19.1?  Cloud based?  What OS are the devices on? 

JSS 11.19.1
Cloud hosted (migrated from on premises several years ago, retained custom URL)
Clients are on 18.4.1 or 18.5

(iPads enrolled and powered on during the summer, now powered on and delivered to the end user)