Maybe the thingy that bolts to the block and holds all the lifters in alignment didn't hold the lifter correctly. I hate when I cannot remember a word and have to sound like the ignorant hick that I am.
Anyway I only mention it because you do not want to miss the root cause and have a bad part do it to you again. Make sure the lifter bore is good too.
Now the next question is "where did the missing metal go?" Hopefully is was so finely ground that it floated to the
oil filter and didn't hang any particles up in a bearing. Is the engine top end stuff that you don't want to take any risk or do you gamble a bit with it? Should the entire engine be disassembled, cleaned, and inspected? Just something to think about. I'm sure we can find people that will argue both sides.