Steps to reproduce
- Go to Tasklist
- In the left sidebar, edit a filter
- In the modal dialog, click on "Variables" in the accordion
- Add a variable
- Click on Save
- Reproduce steps again until including step 3
Previously saved variable is not shown.
Previously saved variable is shown.
The variables are stored as properties of Filter in tasklist:
When the filter is fetched FilterEntity#setPropertiesInternal() is called. 
Then JsonUtil.asMap(json); loses the variables because it doesn't handle the nested collection correctly.