The outcome you got is not necessarily normal, nor is it unexpected. If you carefully read the license agreement, you will see Apple is not responsible for data loss or malfunctions during updates. This is why both the update documentation and documentation across the internet says backup your data before performing any update.
A clone backup, is generally the best to do prior any update, because you can test boot it, and test that it behaves exactly like the original:
Backup now, or forever hold your peace - … - Apple Community
Apple already also considers 2016 Macs legacy:
Obtaining service for your Apple product after an expired warranty - Apple Support
which means, the odds are higher that something could go wrong in the process.
For one thing batteries are only known to last 2-5 years:
Batteries - Apple
Sometimes older equipment does get out of warranty coverage, if a general problem is noticed:
Apple Service Programs - Apple Support
While you may choose not to upgrade, important security precautions that have been addressed with newer software may not be available on older software.
I have also written a user tip so you can decide if you can continue using older software:
https://discussions.apple.com/docs/DOC-250003586
Note Catalina and higher are 64 bit only operating systems, requiring upgrading all software and drivers if you are using anything higher than Catalina.
I hope the service center can break the loop for you.