A/V sync problems are noticeable at about +/- 0.1 sec. The samplerate clock in your phone and camera would have to be exactly the same speed to within 0.1 sec over 10 mins, or however long your clips are, for you to be able align at only one point.
0.1 / (10 * 60) = .0001. That's about 1 part in 10k. Wikipedia says quartz clocks are often as good as 6 parts per million stability. The stuff I skimmed in that and this indicate that it probably costs a bit extra to get the clock frequency that close out of the factory. (and the 6ppm is the kind of long-term drift you can expect around the starting point, not the absolute accuracy of typical gear.)
Keep in mind, most cameras, esp. ones primarily made for still-image photography, don't need a clock as accurate as a wristwatch, so they probably don't have one. Because that would cost extra.
I saw exactly this kind of drift between the audio from my Panasonic Lumix and audio recorded at the same time on a Dell laptop. See Sync separate audio to video+bad-camera-audio, free NLE recommendations, where I describe a slow way to manually sync clips, and ask for a quicker automated way.
Anyway, this lack of clock sync didn't surprise me at all. It's nothing to do with frame dropping, or bad quality gear. It should work fine to assume that both clocks involved each run at a constant speed, just not at the SAME speed. So you can correct a clip with a linear 2-parameter correction: offset + stretch.
I agree with AJ Henderson's suggestion that it's probably the camera's clock that's less likely to be right. Phones have GPS receivers for timekeeping, and may or may not use that accurate timesource as the clock for their audio input samplerate. And also as AJ points out, need to do wifi, which probably requires calibrated clock sources, unless they can just lock on to a received frequency. Not sure how big a deal accurate clocks are with modern wifi modulations, but probably a big deal.
I would have expected issues recording on any two devices that don't derive their sample clock from the same timesource. Until portable GPS receivers were available, movie studios had a master clock that distributed time to all A/V recording devices, so 1 real hour of footage from one camera would have exactly as many frames as 1 real hour of footage from another camera.