If this isn't a bug, it looks like one because it doesn't pick up all styles, just those not specified in the editor styling.

The form this editor is in was created using DMXZone CSS Form Designer.

Hello Ben,

they will sure inherit since they lack their own style. However that is easily fixed. Just make the CSS statement that styles the fieldsets more specific by setting it like :
form > fieldset {}
or #formname > .fieldseClass {}

This way the style will not be inherited.

Regards,