That is interesting. I can't say what is going on but it seems like a bug to me.
XFA Spec 3.1 states:
A Boolean flag which is both readable and writeable. This flag is cleared by the XFA processor at the beginning of an action such as submitting data to a host. Setting this flag at any time before the action is consumated causes the action to abort and a cleanup process to take place. Once the action has been consumated the effect of this flag is undefined.
This flag may be set either by script or by the XFA processor. Scripts must not attempt to clear this flag.
I can't locate a copy of XFA Spec 3.3, which would relate to Acrobat/Reader X. This may be because the version of LC Designer supporting XFA Spec 3.3 hasn't been released yet.
I also have an example that works in Acrobat/Reader 9.1, but not in Reader X. The cancel action is wrapped in an if statement.
This example uses the cancelAction to prevent printing if mandatory fields are not filled in. http://assure.ly/ej9Syg
Note that normally the validationState will clear the red highlight as information is typed into the fields. However in Reader X the red highlight is not cleared. In addition if the print action is cancelled, then the print button ceases to work.
I would be inclined to report as a bug, unless someone else has a better handle on this. https://www.adobe.com/cfusion/mmform/index.cfm?name=wishform