@indigo Yes, I have just noticed this. But in my case, I make a copy of the installer after it's downloaded & before I start the installation.
You mentioned your Time machine restoration didn't include the OS? Did you make partial or full Time Machine backups? I have always tried to make full backups, a few years ago TM was cranky on one Mac & only did partial ones for about six months before a new OS install fixed it up.
// @matigo