How do hydraulic flat tappet lifters fail?

Several of the lifters look terrible.
Hard to tell from the angle the pics were taken from, but I’d say there was a crappy looking lobe to go with every one of them.
For only 600 miles..... none of them look “good”.

My suggestion for your next foray into it would be to not use a fast rate .904 lifter design cam, get genuine Hylift Johnson lifters, and use Driven BR break in oil for the initial break in, along with the black moly paste cam lube for the lobes, dist gear, and bottoms of the lifters.

Then be sure to check the lobe taper and lifter rotation prior to final assembly.

As bad as that one lifter/lobe is in only 600 miles...... it pretty much had to have started to fail on or right after start up.