While editing in Music Maker 15 Premium I have experienced numerous instances of the content of a wav file, already in place in the edit window, being replaced (sound only) by the instrument or sound effect of another wav file after dragging another file into the edit window.
Are you sure is it Wav file? I don't have any problem with this kind of file.
But, when you use loops created with synthiser using Midi, like Vita, in the soundpool, the tag is a note in place of a speaker for a wave file.
For example in the soundpool "rock alternative", if you drag on track 1 :
1 - drums, Break F
2 - guitar, hypnotic 1 on the same track
the first loop "Break F" which is previously a drum will sound like a guitar.
If you click on the right arrow of Break F, opening the contextual menu and selecting the option "instrument editor" you see that the instrument is now a guitar in Vita, the last one. I you turn it in drums, all the line turns in drums.
In fact if you don't use Wav file, on a Midi track it seems that you have one and only one instrument associated to the track, which seems quite logical.
Are you sure to don't be in this case? It looks like.
Try to use one track per instrument for loops with synthiser.
On an other hand to confirm my supposition, answer to the question asked by Procyon.
To have one synthiser or Midi instrument per track is not a large constraint :
. generaly you place your intrument in the space (pan) and in the reality you don't have two musician at the same place or on the same chair on a stage.
. you have rarely more than 10 instruments and when you manage more than 15 instruments you begin to have a very large project... and it begins to become complex. Sometimes by facility, it is easier to split one instrument on several tracks.
. With Wav file you don't have this problem, but it is better to place your instrument in the space : one instrument = one place = one track with its related pan. But you can move the pan.