Seems a bit complicated splitting, filtering joining then splitting again, when you can use the switch node to reconstruct message sequence when removing array elements.
You should be able to split filter and send to ftp using one split node.
e.g.
[{"id":"aee9c356815ed3fb","type":"inject","z":"b9860b4b9de8c8da","name":"Download NB PO","props":[{"p":"payload"}],"repeat":"","crontab":"","once":false,"onceDelay":0.1,"topic":"","payload":"[{\"name\":\"one\"},{\"name\":\"NRNZ_NZ_HL_PO\"}]","payloadType":"json","x":580,"y":480,"wires":[["239a73431c30c19f"]]},{"id":"239a73431c30c19f","type":"split","z":"b9860b4b9de8c8da","name":"Splt to individual files","splt":"\\n","spltType":"str","arraySplt":1,"arraySpltType":"len","stream":false,"addname":"","x":800,"y":480,"wires":[["203566a67c35f5f3"]]},{"id":"203566a67c35f5f3","type":"switch","z":"b9860b4b9de8c8da","name":"Select File Type","property":"payload.name","propertyType":"msg","rules":[{"t":"cont","v":"NRNZ_NZ_HL_PO","vt":"str"}],"checkall":"false","repair":true,"outputs":1,"x":1020,"y":480,"wires":[["25cc2c197580df6c"]]},{"id":"25cc2c197580df6c","type":"function","z":"b9860b4b9de8c8da","name":"Set filename and localfilename","func":"msg.operation = \"get\";\n\nlet lclFolder = '/media/share/sftpSAPTest/NB-SAP/Inbound/Staging/';\nlet srcfolder = '/outbound/';\nmsg.localFilename = lclFolder + msg.payload.name;\nmsg.filename = srcfolder + msg.payload.name;\n\nreturn msg;","outputs":1,"noerr":0,"initialize":"","finalize":"","libs":[],"x":1250,"y":480,"wires":[["e3b27537257158c1"]]},{"id":"e3b27537257158c1","type":"debug","z":"b9860b4b9de8c8da","name":"debug 11","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","statusVal":"","statusType":"auto","x":1260,"y":540,"wires":[]}]
I do not have your ftp node installed, so guess as to its output. Maybe you could supply the ftp output in an inject node, if you require further help.
[edit]
You may also want to rate limit the split with a delay node set to rate limit and only output next element once the ftp node has finished each job, you can set a high rate time limit and use msg.flush to send next element. as in this example
