Most CV's I come across now average 18 month stints before a bump in level and change in company.

That is not them being untrustworthy, it's signalling that they will move if they are not recognised for their work and growth.

Generally it takes 3-6 months to get productive at a new role, another 6 months to generally show competency / growth, and the last 6 months is spent waiting for recognition that you are willing to move up.

Unfortunately the most common growth structures dictate that people need to "act" as if they're in a more senior role already, alongside doing their existing one, and maybe they might get the title and salary bump. Often they don't, or are promised at their next review. Most just move.

If there's a group of 3-6 month (or shorter) spots, then I'm certainly asking about the work that happened during those.