Does it show any message about it safe to switch off, if yes, its software, check Acers website for any updates to your recovery software.
If not, then its either your psu or motherboard not sending/receiving a switch off signal, RMA to Acer and tell them that it happened within the 28 days replacement warranty period and that you have already reported it to support line, this will stop them insisting on a repair and not a new unit swapout.
Stick too your guns and don't accept a repair.