Has something changed in the ID3v2 standard lately?

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ID3v2 support is a bit of a mess to implement.

You can try opening up the file in a hex editor and check if the first three bits are "ID3".  That's where most simple ID3v2 readers would look, but the tag can be located elsewhere in the file.

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I think that Amplifier's playlist can't show ID3v2 tags, but then if mp3 has both v2 and v1 tags, then information in v1 is been shown. Sometimes both artist and song names are mess, but sometimes only other one. Have people stopped adding v1 tags? But I'm just guessing... should do some researching

v1 data is stored at the end of the file in the last 128 bytes.  Look for the word "TAG" 128 bytes from the end of the file.  If it's not there or in the wrong spot that may be it.