A question about controlling the visibility of the text created by osgText::Text on an earth generated by osgEarth

[XianranZ]

Hi, everyone. I created an earth with osgEarth2.7.0 and OSG3.4.0 on the Qt platform. When I add some text on the earth using osgText and try to control their visibility, some strange phenomenon occurred. it seems that the visibility is controlled by the scale of the earth.
Although I set the visibility to true, I should zoom the earth into a smaller scale to see the text.
I don't know why? Anyone knows?
Here is my code for adding text:




Here is my code for controlling the visibility:

Thanks a lot if you can give me some hints!