Is the 8:50 Waterloo-Reading train considered off-peak?

Is the 8:50 Waterloo-Reading train considered off-peak? It shows up on the website and ticket machines as an off-peak ticket, but the gate guard wouldn't let me in this morning, even though my off-peak ticket was bought specifically for the 8:50 train. They forced me to buy an anytime ticket. This was extremely frustrating. Please clarify.