Hard to tell, but possibly because the conditions for capturing the GPS position are exceptionally difficult during swimming. The watch is above the surface only for a very brief time, during which it has to receive and process the GPS signal. It does not receive any signal when submerged. Not every watch is equipped with a sufficiently performant hardware to allow it. Anyway, you can see the list of devices compatible with the OW swimming in the following document, and Venu is indeed not among them: [Which Garmin Watches Can Record Swimming Activities?]
