It seems there is an issue when the control is disabled and the font has regular style. I logged it in our public issue tracking system and you can find it here: http://www.telerik.com/support/pits.aspx#/public/winforms/12408
Meanwhile, you can use the NodeFormatting
event in the following manner to work around this case:
sender, TreeNodeFormattingEventArgs e)
I updated your Telerik points for this report.
All the best,
the Telerik team