Rests and Notes move over stave

[Musikschule - Mufab]

Hi everybody, being a newbie to vexflow, I'm going crazy over this issue. 

I'm having a one bar voice. In empty state it has 4 quarter note rests.

When I insert 4 sixteenth Notes on the first beat, the quarter note rests move backwards on the stave. 4 16th use the space of the half bar.

I cannot figure out how to avoid that. I'm looking for fixed Note positions as long as there is enough space for them. Also the voice expands in length the more notes I insert.

Any suggestions?

Thank you for your help

Horst