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Why would phones use the GPS date over the cellular-provided date?

The GPS unit needs to figure out the locations of all the satellites before it can use them to figure out your positions. The satellite positions are expressed as a function of time, and the times used are measured in seconds within the current GPS week[1].

This is the mechanism that I'd expect someone messed up. I'd expect location errors more than time errors, but because you can get time out of GPS then someone probably has.

[1] https://gssc.esa.int/navipedia/index.php/GPS_and_Galileo_Sat...

The week number is (somewhat) used for navigation, but the way it is used is not affected in any way by the rollover, and there's not really a way (even taking into account bugs) that not knowing what the current GPS epoch is could affect navigation. This is purely a matter of an issue with the time that's output for human consumption, the GPS functionality itself doesn't care in the least.

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