To be honest I never thought about that as I never travel the entire route.
Looks to be the case though as the train terminates in Exeter.
Just found this now though which backs it up...
Advance Purchase fares will also be reintroduced on the route for combined bus and rail fares from Wednesday next week. The company has been unable to offer Advance Purchase tickets – which are tied to specific trains - since Wednesday, as we were unable to guarantee which trains would be running.
Can only presume they've pulled all the tickets for trains heading that way, not the part which is affected.
This paragraph unfortunately doesn't help me either as I can't afford to buy a batch of tickets in one go and hope to claim them back later, nor can I say I frequently purchase them at £X.XX, as I'm just starting to commute on this route - ah well.
“No-one should lose out because of this. FGW's promise is that anyone who has been unable to purchase an Advance Purchase ticket because they were removed from the system and paid more than they would have done normally, will be reimbursed the difference.