Ticket office just assured me it would be fine, even when I mentioned the specific train not stopping at the midpoint
That certainly
isn't the rule, but it
may be the policy of a particular TOC, however unless I got that in writing, I don't think I'd be trying it (unless I asked the guard before boarding - where there is a guard, that is!), or unless I bought the ticket for immediate travel so I could refer any RPIs to a specific person at the ticket office. (and not just say 'someone said it last week' or whatever)
I once asked for a Boundary Zone 6 to Ipswich, not realising it does not exist, and was sold a ticket from Harold Wood. As the ticket was for immediate travel and the TOC providing the train was the same from which I had bought the ticket, I just boarded anyway, the guard accepted it.
I was also sold a ticket from West Drayton to Reading rather than from Boundary Zone 6, from a FGW guard, despite being diverted via Staines. I was slightly concerned about the way back but decided not to worry as the chances of a check on SWT are minimal and I had asked for the right ticket and had clearly used the ticket on the same route on the outward as the other route was closed, and as the guard issued it, then if there were any problems the guard could then be contacted.