Description of the bug:
In the .mptm format, Random Variation of Volume causes notes to slowly decrease in volume over the course of a long pattern, but only for notes that are not assigned an instrument (after a channel has been assigned an instrument).
How often does it happen?:
Always, given the right conditions, but randomly (not sporadically)
How to reproduce (step by step description):
To hear the bug, create a 128-row pattern in an .mptm song. Create an instrument with an attached sample, such as a kick bass. Set the Random variation slider to halfway or above to notice the difference in volume randomness. Step-record the note every other row in the pattern (8ths), but delete all instrument references to each note after the first row, so that only the first row assigns the instrument to the note. Now play it back. While there are variations in the volume, the notes tend to decrease in volume over time, sometimes rather quickly.
Shouldn't the randomness be acting on the original given volume rather than the previous adjusted volume? Check the Pan settings to see if this also occurs with those random variations.