I am sorry, but I can't find an answer for this anywhere. I have used VPP invitations for years, once a student accepts it, I can push iBooks to their iPad. I have also always used the self service portal for apps and removed the app store.
In the past, a student would receive the invitation in the self service portal, register in the app store (which would open just for a moment to accomplish this), and done.
This year, they get the invite, click Register, the app store opens for a split second and closes much too quickly for them to agree to anything.
Thoughts? We are up to date on our JAMF console and use ASM and DEP.