Thanks for the report.
This issue is interesting because it's a design issue. With current implementation, closing days are really closed. They do not even exist in the internal modeling. So it's really impossible to have something starting or ending on those days.
So we cannot fix this issue right now.
However, the good news is that we have started a new implementation which will overcome current limitations of the Gantt Diagram (for example, using hour/minute/second/millisecond in time). This new implementation will also fix this issue. But you have to be patient...
We'll post a message here in the incoming months when this will be fixed.
Sorry for the delay!