I had the same issue in January with a late departure from SIN (having originated in BKK). In my case I was proactively moved to a later flight. All on an NZ-issued single PNR.

However, I was unable to make seat selections on the revised domestic flight using the app or web at any stage until after all arrival procedures and turning up at a domestic kiosk.

This seemed to me me to be quite a limitation, and it was almost as if I was ‘locked-out’. I have wondered if originating in BKK on SQ (NZ-codeshare) somehow disables full functionality of the app and web - almost like the systems saw my AKL-WLG as a kind of codeshare.