Hello Tarmak,
It is a bug. The next Visual Dialplan release will have this bug fixed.
In the meantime you may try to deploy the dial plan version 1.6 to Asterisk server version 1.8 since there are no major changes in the dial plan structure between these two versions.
Of course, there are several new functionality introduced in Asterisk 1.8 but the dial plan developed for Asterisk 1.6 should work with Asterisk 1.8, so please feel free to deploy your 1.6 dial plan.
Michael Denich
Apstel support team