(US perspective to this answer)
No, it's not normal.
However, it's also not ridiculous or anything. Unlike PhD degrees, which principally prepare you for research, MSc programs are often more like professional programs.
It's weird to do a second PhD (even though many people seem to ask about it on this site), because most of what you learn in one PhD ("how to participate in academic research") is transferrable to other areas, and PhD positions are often funded by governments and universities to train another generation of scientists. Those funders don't have in their mission statements to train and retrain and retrain people who can't decide what field they want to be in.
I think the scenario where you'd most likely see someone do a MSc after a PhD would be to do a career shift to a field in which there are industry positions. For example, a biology PhD might do a masters in statistics/data science or engineering/computer science if those degrees would help them get a job in those fields. It is not a path someone would go down on the academic career path.
A MSc does not seem like a good path for someone interested in a hobby project - masters programs do not typically come with funding, and are quite expensive. There may be continuing education opportunities to let someone take individual courses towards a hobby project for a reasonable fee rather than enroll in a full degree program.