I will now point you towards mainland China, where kids as old as 90 will shit in the streets. I’m not kidding either (although this is changing in the larger cities.... slowly)

Yeap. I've seen this. People who pee or poo wherever they're at.

That’s learned behavior.

But isn't this just an instinct not to defecate at the place where he/she plays and spends time? How can you be so sure that it is about privacy?

A private place is often a good idea, so predators wont eat you when you are so vulnerable. There are many places where nakedness is the norm and the naked people will probably still search for private places when defecating.

Are you sure that's not learned behaviour? I've only ever seen it in older kids. I'd hypothesise it follows "expert" advise that suggests what I consider very late potty training.

Well consider 1 year olds will do it (my daughter did it when wearing diapers) even though we have no indication it was needed.

I've spent time around more babies & toddlers than most people see in a lifetime (sometimes >50 per day; probably no more than 2000 per year though; for >decade; [working at baby & toddler groups]).

Could you say if this was a fixed behaviour (ie they always sought a private place to poop), had a fixed location (eg always behind the couch), if you used a potty at the time, and exactly which month it appeared at so I can note it with my own observations. Cheers.