They intentionally chose a Qualcomm QCM6490 for the FP5 instead of the rumoured Snapdragon because Qualcomm will offer longer support for that chipset (which is supposed to be used for IoT/industrial applications). We don't know what chip is in the new one yet, all we have is rumours, but I wouldn't be surprised if they pulled the same "Snapdragon but industrial" trick here.
Many mainline supported SOCs are unavailable to a company like Fairphone, which only produces a tiny amount of phones (less than 50k for the latest and greatest model). CPU manufacturers aren't going to waste time sending their top-end chips to some small company when Samsung can pay more per CPU and can take shipping containers full of them. That's also why F(x)tec phones come out with such outdated processors. Small companies will have to make do with whatever niche products are for sale in low quantities.
Many mainline supported SOCs are unavailable to a company like Fairphone, which only produces a tiny amount of phones (less than 50k for the latest and greatest model). CPU manufacturers aren't going to waste time sending their top-end chips to some small company when Samsung can pay more per CPU and can take shipping containers full of them. That's also why F(x)tec phones come out with such outdated processors. Small companies will have to make do with whatever niche products are for sale in low quantities.