It is surprisingly common even on desktop monitors, though not high-end ones.
The panel is something that will often get changed over time in given laptop model without any visible change in the model number etc. and there have been cases of early versions (including review models, of course) having better panels and later revisions having 6-bit ones. I'd be really irritated if it happened in a high-end model but I'd not be surprised to discover there are examples where it has.
They are very careful not to state parts of the panel spec that they don't absolutely have to, then they can legally (though obviously not morally) change the unstated parts without any issue. With early review units there are other extra excuses available to them.
It is rare that a screen will get such a significant downgrade, though it has happened. More common is downgrades in RAM and drive performance. Drive makers themselves play fast and lose with this sort of trick, sometimes changing both controllers and memory to the detriment of performance multiple times without outward change in product codes.
The panel is something that will often get changed over time in given laptop model without any visible change in the model number etc. and there have been cases of early versions (including review models, of course) having better panels and later revisions having 6-bit ones. I'd be really irritated if it happened in a high-end model but I'd not be surprised to discover there are examples where it has.