Using update-manager the changelog appears to only show the latest entry.
Using Synaptic - the rest of the changelog is shown.
ProblemType: Bug
DistroRelease: Ubuntu 19.10
Package: update-manager 1:19.04.7
ProcVersionSignature: Ubuntu 5.2.0-9.10-generic 5.2.1
Uname: Linux 5.2.0-9-generic x86_64
ApportVersion: 2.20.11-0ubuntu7
Aptdaemon:
Architecture: amd64
CurrentDesktop: XFCE
Date: Tue Jul 30 19:48:04 2019
InstallationDate: Installed on 2018-08-27 (337 days ago)
InstallationMedia: Xubuntu 18.04.1 LTS "Bionic Beaver" - Release amd64 (20180824)
PackageArchitecture: all
SourcePackage: update-manager
UpgradeStatus: Upgraded to eoan on 2018-10-14 (289 days ago)
modified.conffile..etc.default.apport: [modified]
mtime.conffile..etc.default.apport: 2018-09-21T16:30:23.845026
I've looked at this it and I think the difference might be due to the programs focusing on different things when it comes to changelogs.
For update-manager you are getting a newer versions of packages already installed. The focus here is on what the changes include, which issues are involved and what you might need to know before installing. Any historical changelog entries are less relevant since they discuss bugs or issues which are already resolved due to the fact that this is already installed on your system.
For synaptic, which is a full-fledged package manager it makes more sense to be able to see the complete changelog for packages to see what has changed since the dawn of time, including when it was initially packaged 20 years ago.
Sot it looks like this is intentional behaviour, though I've also added synaptic as affected, so we might get a more official confirm or deny.