I guess, there is some correlation between being worthy of YC club and successfully negotiating yourself a founder title. That's probably one of the reasons, why somebody with a non-founder title can't get in that easily.
Saying that, such situation is likely to has negative outcome for the company. A conflict in the founding team and high likelihood of significant contributors leaving the company and abandoning projects can't be good.
IMHO, anyone who has been around a company since the last major pivot (significantly different product, market or business model) should deserve the founder title.
I am employee #1 at the startup I'm at and I've been here since we were a completely different company, targeting a different market with a different product. Being present before these major pivots, means you are very likely to make major contributions to the new direction of the company. This in a way makes you a founder. You may not be the founder of the legal entity, but you are a founder of the company insofar as product, market and business model are concerned, since all were conceived with your involvement. It's very frustrating to be on the other side of the founder-employee divide.
Saying that, such situation is likely to has negative outcome for the company. A conflict in the founding team and high likelihood of significant contributors leaving the company and abandoning projects can't be good.