I don't see the stated problem. The music listed loops back to a non-zero frame in each case, and still properly continues to play the music in the frame that it jumped back to. No bug?

Tracker music is... keeping track of "notes" it play (they are actual notes but also contain some other info).
Those notes can also change the pitch, volume, playback speed and may force music player to jump into certain position, that's why it does not work as your regular mp3 player.
For example, if we want to know the exact BPM for some point in time, the only correct way of doing so is to play the music from the beginning which consumes a lot of processing power and will cause lags, that's why the synthesizer does not try to calculate these values when you skip some part of music.

So, let's say, you are playing some music which changes its BPM / pitch rate / jumps to some position in its list of notes at 0:10 (10 seconds from the beginning). If it plays for 5 seconds and you skip to 0:15, the synthesizer will skip this event at 0:10 which may be important for the playback of the future notes. It works in other way too - if you have played for 0:15, this event (which happens at 0:10) has affected the synthesizer (it has instructed the synthesizer to play with certain speed / pitch etc) and if you skip back to 0:05, it may affect the playback too (apply the effect sooner than needed in this case)

Short answer - it is not a bug, it is a feature!