This is usually described as a halo or ringing artifact around high-contrast edges.

The most common cause is image processing, especially sharpening or local-contrast enhancement such as unsharp masking. Phones often apply aggressive processing to JPEGs to make images look punchier, which can create bright outlines along edges.

It can also come from frequency-based processing in general. JPEG compression and some filtering methods can produce “ringing” near sharp transitions because abrupt edges are hard to represent cleanly.

A third contributor can be the sensor pipeline itself: most phone cameras use Bayer-type color filters, so the image processor has to reconstruct full color from partial color samples. On very fine detail or sharp edges, that reconstruction can introduce edge artifacts.

So yes: it’s generally unwanted, and in phone photos it is often a side effect of processing rather than a scene property.