Aaah. Yes. this is V13. Core POS behaves this way. On closing of the voucher it passes the entries. Up till V12 the entries are passed per invoice. You may try POS Awesome from here . It works on V13. Updates per invoice so inventory is deducted in real-time.
Other option is go to V12 and use POS Bahrain which works in offline mode. Updates sales invoice and inventory every 3 minutes. POS will work if internet connection is broken with server.
POS Invoice is a bill which send information to make Sales Invoice. Thats normal behavior of all retail POS.
Same issue highlighted here.