Vertical positioning of lyrics is affected by changes to other staves.
I consider this a minor GUI problem which occurs randomly, yet frequently.
Steps to reproduce:
1) Create a score withy a vocal accompanied by several instruments. (I have not experimented to determine how many instruments are necessary to see the effect.)
2) Populate the score, starting with the vocal staff, including the lyrics.
3) Observe the position of the lyrics as the instrumental lines are populated or make changes to an instrument part. I observed the problem in the attached score while adding dynamics to the brass and the conductors parts at measures 63 through 67.
I consider this minor bug since I have not had a problem when I create the individual parts and the positon of the lyrics in the score is not as important. However, I only do minimal editing to the parts, mostly small formatting adjustments, so I cannot be sure the problem could not happen with only a vocal line.