Symptoms

Problems with the "Cancel changes made to the row on the editable list" action
 

Diagnoses

After the cancel changes method is called, it seems to prevent reading smart object data about a selected row. So when attempting to edit any existing line, the fields are all shown blank. As well as this, because data about a selected row can't be transferred using the smart object fields, this is stopping other form functions that depend on that data working correctly. This is not an essential feature, but the method does seem to be the only way to cancel new rows or undo changes to existing ones, so having it working improves the form usability.

 

Resolution

I've had a look at the cancel line button rule, and it seems the problem may have been to do with the list refresh action which was called with the cancel changes action.

After disabling the action, the problem doesn't occur anymore. The list refresh action wasn't actually required, so I've removed it and the form seems to be working correctly now.