When I click on the button, the font size shrinks to 12. However, the result is :
Finally, I found the reason/solution!!!
This is a known bug for Android 3.1+
Issue 17343
Issue 22493
Possible workaround are:
text.setText(text+"\n");
or
final String DOUBLE_BYTE_SPACE = "\u3000"; text.setText(text + DOUBLE_BYTE_SPACE);