Just wanted to share that we have seen issues doing the upgrade as posted in the subject.
In a none mixed vc mode stack we have not seen this issue.
In a 1 member switch mixed mode vc we have not seen this issue.
Quick work around. Upgrade the backup RE then do a switchover , and upgrade the former active RE .
If possible set your vc mode to none mixed before applying the upgrade.
On my of our stacks we were able to do the Junos Recovery to the previous Junos release 19.3 and 19.2 and do the upgrade from there.
Now saying that with so many factors that ymmv.
We do not have a JTAC case open for this as we are waiting for our contract to get renewed.