How do hydraulic flat tappet lifters fail?

For it to have failed with that few miles on it, to the degree of wear that you’re seeing now, it basically had to start failing during break it.
It starts as a tiny spot where the lifter and cam exchange a little material before the two surfaces have created a work hardened wear pattern.
Once that little surface defect is created, it’s just a matter of time before you end up with what you have there.
This is why the very first minute or two of the break in process is so important........ to make sure that exchange of material between the cam and lifter never happens.

When you remove the cam, see if there is evidence that other lobes are starting to go away.

Did you run the Lucas oil the whole time, or just for break in?

Was that cam a fast rate .904 lifter profile?
Those less tolerant of lifter bore misalignment.

I’d check the lobe taper on the lobes that are still good to verify it’s correct (you want around .002” taper).

I’d say these days, the most common reasons for a flat tappet failure of that kind is going to be from lobe taper being incorrect, incorrect crown on the lifters, or incorrect oil being used.

A few years before the whole lifter debacle situation happened(early 2000’s), I had an opportunity to see first hand how the wrong oil could impact a flat tappet cam when stiff springs were being used.
I had built a 440-6 for a customer, Crane hyd cam, Hylift Johnson lifters, Isky dual springs, HS rockers.
Dynoed the motor, no problems with break in.
There was no “break in” oil then as cam failures weren’t very common(or at least less common that now).
Motor was run with VR-1 20w50 for a few years, no issues.
The car got driven quite a bit....... it probably had 15,000 miles on it.

The owner reads some stuff on line about oils....... decides synthetic would be better for it.
Puts mobil 1 10w30 in it.
Within a month the cam has several flat lobes.

Nothing against Mobil 1...... I use it in my DD vehicles....... but gas mileage formulated oils aren’t really a good choice for your old school muscle car with a flat tappet cam and big springs.

I’m just pointing out, that they can still fail long after they have been put into service.