Thanks, everyone. I can post system info when I'm back at the machine.
@ BanditDD - not sure why you'd think I would want to "deceive" anybody. I have a genuine question because I'm not tech- or Mac-savvy enough to understand what's happening.
@ Mike Boreham - Yes I previously ran the machine with Sonoma on OCLP. Due to some software incompatibility issues I eventually wiped the main hard drive and clean installed Ventura via Recovery (i.e. last officially supported OS version). After a few weeks I eventually got the Sequoia upgrade prompt. There couldn't have been any OCLP "leftovers", the hard drive was completely blank, and I did not use TimeMachine backups to re-install previous User, System or other related settings or files. It's basically a "new" machine. Yesterday I also downloaded the latest Sequoia Installer (via MIST), just to check if this was some fluke - and it downloaded and installed again, without issues.
This may sound silly, but is there any way that the machine got hacked / remote pirated or something......?