Fairly common for startups that go through multiple rounds of funding.

If you invest during a seed round, chances are the funding is much less than $50M. Series A usually is much less than $50M too. Series B or C might put you over the limit, depending... but that doesn't disqualify the earlier purchases.

Meeting the holding period could be easy or hard, depending on what the company does. If it takes 5+ years between when it hits the $50M limit and when the shares are marketable, most holders will have a qualified disposition. If it's acquired and the merger terms aren't well tax managed, that may be a disposition for all holders and that sets the holding period. If it becomes marketable quickly, then some holders are likely to sell at least some shares before meeting the holding period... Avoiding capital gains tax is nice, but not nice enough to forgo realizing gains when experience has shown that stock prices can drop rapidly for a variety of reasons that may be hard to forsee.