Ask HN: Does esop's allotted to employee's undergo dilution?

I have a naive yet important question(just because these things are talked so less about, or subjective) around employee stock options.

Does it ever happen(even exceptions) that allotted employee stock options to an employee doesn't get diluted(somehow) over next round of fundings(given secondary sale, i.e dilution takes place for founders/investors/rest) ? If yes, how's that(any reference would be helpful) ?