I wonder if the recent news and rumors about Fuji playing around with just about everything other than Sony sensors are for exactly the reason Bernard states? The sensor in the X-T3 is unlikely to be a Sony (Sony wouldn't bother making one 26 MP sensor for a relatively low-volume customer when it's not meaningfully different from a 24 MP sensor). If it were 30+ MP, or if it were 24 MP but BSI and possibly stacked (memory directly behind the sensor, not a Foveon-type RGB sensor), I'd say " mildly interesting, new Sony sensor variant", but a couple of MP different suggests someone else... Then we have the recurring rumors about Fuji, Samsung and Panasonic in various combinations designing more radical sensors - organic, global shutter, heavily video-optimized, etc.
This all makes sense if Sony is slow-walking APS-C sensor development because the market is becoming "$400 cameras and Fuji". If you want to sell a finished camera for $400 ($500 in a kit with a terrible lens), which is what a Nikon D3x00 goes for, you probably can't afford to pay Sony more than $30-$40 for the sensor (someone like Edmund who knows far more about the chip market than I do, feel free to correct me). Fuji is building $800-$1800 cameras, and not only can they pay more for the sensor, they'd like to if it bought them better sensor-level performance. Sony may not want to build a higher end APS-C sensor that can't sell into the $400 market just for Fuji, though.
Of course the wild card is some new Sony APS-C body above the entry level - it would almost certainly feature a new sensor. It might be a new 24 MP variant, or it might be a resolution jump. If it's the 26 MP sensor, I'll be surprised that turned out to be a Sony design - it's possible that some new technology led to an accidental resolution increase (what if the space between pixels was reduced, so the same pixel size led to a slightly higher resolution sensor)? Whatever the new Sony might be, Fuji probably has access to it.
If Sony doesn't release more APS-C bodies, Fuji's other hope for Sony sensors is Nikon (big enough to get Sony moving, especially if Fuji also wants the sensor) releasing something in upper-end APS-C - potentially mirrorless. Nikon doesn't want to release a $1300 Z5 and admit "well, it's actually the D3500 sensor in a nice body"... Apart from that, Fuji needs to be sensor-shopping, and they seem to be starting.
So... in reality, in the way CMOS sensor manufacturing works... it is not a Sony problem, it is a Fuji problem. For Sony the manufacturing costs and benefits are the same.
So, making sensors from mobile phones to the Phase One 150 MPx monster... it uses the same manufacturing lines using the same processes. It all starts with a silicon wafer if 300 mm of diameter than then it is cut to the size of the chip... if the chip is bigger, you get less chips per wafer... higher costs per chip... (that also joins that it has lower yields the bigger the chip... it is more easy that a defect in the purity of the silicon wafer or the process ruins a chip than if you make it smaller... you get in percentage more good chips per wafer and a higher yield).
Usually new technologies like BSI and stacked sensors are teste first with smaller chips... because yields are higher and the probability of screwing something is higher... so you at least get some good chips per wafer and recover some of the money... that it is the reason you saw first things like double ISO or BSI in mobile phones than cameras.
Now, Sony produces some standard designs... like the typical 24MPx that has been used a lot of cameras... but also allows other companies (and Sony imaging division) to custom fit their designs. D850 sensor is a clear example, they used Sony libraries to design a sensor and Sony is building it... Nikon had to pay much more for it... since they can’t share the design cost and fine tuning of the manufacturing line until getting a nice yield with other camera makers, like in the case of the 36 MPx sensor used in the D800, A7r or Pentax k1.
So, Fuji could come to Sony and do the same, design from scratch their sensor using Sony fab technology... they just have to pay more for it... (although the whole year there is the rumor Sony will release an update to the ASP-C cameras, specially to compete with Panasonic and Fuji in vídeo with 4K 60p... if that happens and the sensor is of 26MPx probably is a shared design)
Now, I will really love that it is a Samsung design... it will make things more interesting
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