What is the express version?
an express patch is a small service pack, a few dozen updates.
a patch is a single update.
The same ISO was used for upgrade, why would one be different?
i am not sure if there was single file in baseline or multiple. I have never seen the build version change when you are using same ISO
If you are going to higher build version from lower then i would suggest install with offline bundle try using UM. you can download offline bundle from my.vmware.com.
its possible may be VUM is not able grab all component for iso. you can "more" the log ESX (patch and update installation logs): /var/log/vmware/esxupdate.log to see what exactly happened on those host.
Why do we need roll back the version:
Ideally if you on lower build and want to go on higher build you need not roll back just upgrade. as latest file include their predecessor
Hope it answer all you questions.
thanks