Fascinating and well documented. The tl;dr: basically some models of iPhone and Watch use particular MEMS 32kHz clocks (instead of larger crystal clocks) which

Fascinating and well documented. The tl;dr: basically some models of iPhone and Watch use particular MEMS 32kHz clocks (instead of larger crystal clocks) which aren't sealed well enough to keep out small molecules like helium. It can stop the clock. A week or so of time to air out, so to speak, will bring the device back to life. Newer versions of the MEMS clock are better sealed.