The "eZ Page swap workflow event" updates blocks so that they are attached to new nodes. However, it does not update block items in the ezm_pool table.

If you have a block that contains Article A (node ID 8, object ID 8), then swap Article A with Article B (node ID 9, object ID 9), the ezm_pool entry will still have node_id = 8 and object_id = 8. Nothing will update the stored node ID and object ID values in the block except removing and re-adding the article. So the object ID is corrupt after a node swap. This results in Article A showing up in the back-end (because the back-end looks up the object ID) and Article B showing up in the front-end (because the front-end block template loops through $block.valid_nodes and thus looks up the node ID).