Steps to recreate issue:
1. Open the online demo ( http://demo.bpmn.io/new) and create a new diagram
2. Double click on the start event to assign a label and enter text "AAAAČĆŽŠĐZ"
3. Use mouse scroller to first zoom-in and then zoom-out the diagram
At this point the entered text gets messed-up (screenshots below)
After editing label:
After zooming in and out: