Thanks for the bug report. Other people have reported the same problem. Here's my response to another user:

The "thumb" keyboard is coming up instead of the standard keyboard because you most likely have the "Launch via rocker key" setting set. This means that any time you press the "Enter" hardware key, the thumb keyboard will be displayed.

You can turn off this setting as follows:
1. Cause the virtual keyboard to be displayed.
2. Click on the menu button on the virtual keyboard, then go to Tools->Settings.
3. In the "Text input settings" dialog, select the "Finger" tab, and uncheck "Launch via rocker key".

The problem should now be solved.

VNC viewer has some issues when used with the thumb keyboard, but at the same time, the only way to use the thumb keyboard with VNC viewer is to have that setting checked.

Please let me know if this fixes your problem.

Regards,
AaronL