They may not be a coincidence but they could be overblown. First there was a rumor that it was overheating so Samsung was using their own in-house tech, then there were rumors that the G Flex 2 was overheating but those were shot down. Now there's rumors that the M9 is overheating. It just seems like the rumor is following every phone that is running the 810 whether it's accurate or not. People are just looking at what's going to come out next: the G4 and Z4. I wouldn't be surprised if they are simply re-stating the same rumors simply because those phones are supposed to be using the 810 instead of them coming from credible sources inside of LG's and Sony's R&D labs.
I would take each one with a grain of salt until the professional reviews start coming out. Those normally take time but they should give real world impressions regarding the performance of each phone along with operating temperatures. Until then, I will keep an open mind regarding the 810.