Yes, in its zeal to implement a single, unified connector, the USB IF failed to recognize that having one doesn’t make everything else go away, or any simpler, just harder to discern, because there are no longer different connector types to accommodate all the various functions USB now covers.
The standards are also in flux, with new versions issued faster than the market can catch up. Those standards can also defy logic, as evidenced by the multitudes of cable types, and allowance for a state-of-the-art cable to not have to conform to the most current data standard. Why don’t all C2C cables support USB 3.0 speeds at a minimum? And then there’s the implementation by the manufacturers, which can be less than perfect.
It’s a classic “by committee” outcome that tries to please everyone, but results in a cluster that can hinder more than help. The practical results are predictable.
The Apple 18W adapter is a well designed unit, as their power adapters are in general, but even they can lack full support, which isn’t evident until newer versions are issued, as with the 20W model, which is required to support their latest accessories. They’ve also updated their 29W to 30W, and another higher capacity model to fill in some functional blanks as well.
However, that’s only half of equation, and as noted above, the device in question also has to play its part, and the possibility that the Xtar may be at fault can’t be discounted.
My experience with one of their chargers has been frustrating, in its refusal to 30Qs at anything higher than 1A, despite verified combinations of various adapters and cables, just via a simple, relatively dumb Micro USB connection that isn’t subject to the issues surrounding PD. Whatever their logic is in automatic current selection is, and the inability to override it, has me wary of considering another.
If you’ve found a solution that works, that’s what matters, but sadly, unexpected issues can occur.