I used to use a Meier Audio headphone amp which ‘fixed’ this with a switch toggle. Handy feature.

Not a fan of the Japanese art of karaoke I see ;-)

It was 1) only a brief period where they were experimenting around with new stereo broadcast capabilities or 2) a schtick

Just that so much good music was made during this time that we imagine the period lasted longer than it did

Yeah, some were pretty crazy.

Is mostly because stereo sound being widely available was novel and new. I have several albums that are literally just made to let people show off the stereo aspect of their new hi-fi.

And there are some old mixes that are fairly extreme by today’s standards but sound like a very real soundstage - the drums are over there, the vocals over here, etc.

But then some took it too far with the control hard pans and then relying on the hard pan to fix bad levels (like that the drums are too loud)

But even today, it’s not unusual to have some instruments entirely (or very nearly) in one side of the other. Just not the primary instruments carrying the harmony and melody.