DMU Class Seat Reservations

[maitias]

Was on a DMU class 18:30 ex Cork to Dublin on Sunday. Something I noticed as soon as we pulled out of Cork was the Seat Reservations notifications disappeared. (Also which I thought was odd was that carriage A was at the back of the train as opposed to the front of the train.)
As we progressed on our Journey I thought the names would reappear at Mallow and they didnt. People got on board and occupied any seat even though I new some were clearly marked reserved from Limerick Jct & Thurles. Some arguing followed where people were saying I had this seat reserved and others said there was no name there. People either moved on or didnt bother to argue when they realised the name was not over the seat. This happened at every station. Is this something that happens on the DMU class after it pulls out of its starting station or a once off where the system was switched off by the host? Quite annoying for people who prebooked a seat and never got it.