I too am not comfortable with the outcome in this case.
Yes a ticket with wrong date was bought in error
But the error was accepted twice, with no attempt to reissue a ticket with correct date.
In my view the average person would therefore assume as they have paid for a journey, and error is being accepted, that whilst not technically correct, a consistent approach to the error is taken.
I struggle with the concept of 2 say it is ok, third (afterwards) says not and you should have a penalty. Had the not acceptable date been applied at the start then I side with the railway on this. If you went to theatre, a member of staff said ok to sit in different seat to your allocated ticket, you wouldn’t expect after the performance to be hit with a penalty because another person subsequently says you weren’t strictly in conformity with the ticket you purchased. So why railway thinks it can is beyond reasonableness.
Someone needs to explain clearly why if there are gates at start and end of journey, both rejecting the ticket, then different interpretation of rules can be applied at each. It’s not customer friendly. You shouldn’t be taking it out on the customer, but should be sorting staff training that allowed the inconsistency.