It goes to show how easy it can be to mess up the sound coming out of mobile devices - so many dependencies on OS and built-in apps and settings.
A modern smartphone on stock firmware using a good music app like Neutron or UAPP will produce a clean, flat frequency response in the vast majority of cases. A niche case of a 5 year old device running on custom third-party firmware is another matter, and can be expected to cause problems like these.
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