It isn't rendering in scanline :) What you are seeing is the effect of the limit of the data packet, which limits how much data the node sends back at once. This means the image builds in blocks, as it receives packets of that size, though on the node regular progressive rendering is going on as normal. Corona simply can't do scanline rendering at all under any conditions :) As a note, once rendering is complete, all the data packets from the nodes will be sent over, so you still get all the results of the (progressive) rendering from all the machines, just received in batches based on the packet size.
You can adjust the size of the chunks in the Render Setup dialog, Corona, Team Render settings by changing it to Manual and entering a size for the maximum packet to be sent at once (see screengrab) - larger values will send larger packets, meaning the image will build up in larger chunks. Again though, it has no effect on how things are being rendered on the workstation or node, and only affects how that data is transferred between the two.
The other parameter in there changes how often data should be collected from the TR nodes (longer will mean a longer waits for results from the nodes to come in, but they will have had more passes done since they'd have run for more time).
