freeze columns make a cell text disappear

Steps to Reproduce

1. new a blank sheet
2. Click toolbar > "merge & center" on C1:F1 and enter abc in the cell
3. freeze column C, Ranges.range(ss.getSelectedSheet()).setFreezePanel(0, 3);
see keikai-588.zul

Current Result

the text in the merged cell disappears

Expected Result

the text is still visible

Debug Information

• if you load a file with the same merged cell, this bug doesn't happen. It only happens when you do it in the above steps.
• unfreeze the columns, the text appears back