Have you reset the iPad after adding it to the prestage? Even if you have not proceeded through the setup assistant, I've found that devices often need to be fully wiped before picking up their new ADE status if they have ever been powered on. And unlike on the Mac side, there is no way to trigger ADE enrollment post-setup on iPads.

Looks like there is Jamf PI on this: PI119894.

Someone on Slack mentioned this:

"We've seen success with leaving devices unassigned for 30+ minutes then reassigning to the desired MDM.  Jamf recommends 24hours." 

Not sure what my comment is worth, but just reporting that we are experiencing this issue, too. This PI was referenced in a support ticket I opened this morning. There's one specific iPad that has been assigned to Jamf (via ASM) for months but somehow doesn't show up in any prestage nor in Settings > Global Management > Auto device enrollment > Devices.

The "workaround" I was given is as follows:

I suspect we are running into a current product issue, PI119894. This is an issue that is caused by assigning it to a new server without unassigning it from the previous server first.
We do have a workaround for this. If you unassign devices from server again and let it set for 24 hours it seems to work when assigned to a server again. Try this and let us know how that works.

I've unassigned the device from MDM in ASM but did not release from the org (did that during troubleshooting earlier, too, to no avail). Will reassign to Jamf tomorrow afternoon then check on Monday to see if it shows up in Jamf.

Fingers crossed.