I've attached a sample project to illustrate the issues.
To make it easier '.activiti' here refers to the diagram files edited with the designer.
bpmn20 is the *.bmpn20.xml generated by the designer.
When using the Mail Task you can set the text and html in .activiti, everything get stored correctly,
but when you change something in the bpmn20 then the content of the text and html properties is dropped when the .activiti file is updated.
See Sample diagram in attached project.
Another issues is with the description property, when it is set in .activiti and changed in bmpn20 then those
changes are not synced back to the .activiti file. Same with the initiator attribute, I suspect others properties
that are both in .activiti and bpmn20.xml are also affected but I haven't tried everything.
See Description diagram in attached project.