I'm fairly confident it's a bug.
You can sprint permanently right now, so long as you don't run into anything or fall a long distance on the horse. If sprinting permanently on a horse were intended, they wouldn't have a stamina bar at all I imagine.
So probably safe not to plan around this bug.
I don't think it's a bug. I think everyone is just looking at it wrong.
The Stamina bar represents the amount of damage you can take while being hit on your horse before you're thrown off. Test this. If your Stamina bar is empty, a single hit will throw you from your horse. It is much more difficult to knock you off when your bar is full. It makes sense to think of it this way, as your stamina bar is instantly full if you throw out a fresh horse.
Just try it, you'll see that it really has nothing to do with the distance it can run.
I don't think it's a bug. I think everyone is just looking at it wrong.
Even if you look at it as an unintended feature, it still makes expanding stamina unnecessary for the stable system which means it is likely that unintended feature will get nerfed in order to make stable stamina increases more impactful in PVP.
It isn't just an interesting theory, it is an accurate one. That is how the system works and I am almost certain it is intentional. I am eagerly awaiting someone to release numbers on how hits with horse stamina works.The Stamina bar represents the amount of damage you can take while being hit on your horse before you're thrown off. Test this. If your Stamina bar is empty, a single hit will throw you from your horse. It is much more difficult to knock you off when your bar is full. It makes sense to think of it this way, as your stamina bar is instantly full if you throw out a fresh horse.
Just try it, you'll see that it really has nothing to do with the distance it can run.
That's an interesting theory at least. So adding stamina isn't about making your mount sprint for longer but letting you take more hits before falling off (i.e. if you're going to run through a bunch of mobs you want to not have been sprinting).
That certainly seems counter-intuitive with how stamina works in general for our characters, but it's entirely possible you're right.