I haven't worked with the path finding system very much, but one thing that might work is forcing the path to recalculate when you change posture.
This way, if you stand up, the solid will reappear, the path finder will be forced to recalculate a path to the last destination given, and the path will no longer be valid.
If you try this method out, for obvious reasons, it's important to make sure that the path is getting recalculated only after the solid reappears. Behaviors and event sheets can sometimes execute their effects in an order you aren't expecting. IF this becomes an issue, you can place the path recalculation action after a wait( 0 ) action, to delay the recalculation to the end of the frame.