Currently the Revit API does not provide a direct method to get sketchplane associated with a face based family instance whos workplane no longer exists. In these cases the family instance is "hosted" by a sketchplane which has no direct relation that can be determined using API.
More deatil can be found in the links below to oringal forum post. The post does give a solution for indirect way of determining the sketchplane but would be much nicer if the API had direct method for this.
https://forums.autodesk.com/t5/revit-api-forum/how-to-move-and-disjoin-using-api/td-p/9527659
https://youtu.be/QceKgq7jBw4
Thanks,
Jon
