I am making a basil-raspberry sour with WLP648 as yeast. Unfortunately I need to go on a work trip for the next 2-3 weeks and the beer has not fully attenuated fully. I was wondering if it hurts to let the beer sit on the yeast cake for another 2-3 weeks before I rack it to secondary with the fruit, or if the yeast would start falling out and settling on the bottom (meaning not enough yeast will be moved to secondary to work on the fruits).
You should be ok to rack onto fruit once you return. If you're worried about not having enough yeast, you could rouse the beer a bit before racking, or add more yeast along with the fruit. If you keep the beer at primary temp though, I wouldn't be concerned about lack of yeast.
You will be fine, WLP648 is such a slow fermenter that many people leave their beer on the yeast for extended periods of time. I am of the opinion that on a homebrewing scale, you can leave your beer on the yeast cake for months. The bretts in WLP648 are typically not very good flocculators, meaning that they wont drop out of suspension for a while. Anecdotally, I can tell you that I have never had problems with yeast counts even when transferring to secondary with fruit after months in primary.