To be honest I think the wording of the minimum fare clause is poor anyway.
From the 16-25 Railcard terms:
4.5. The 16-25 Railcard has a minimum fare that applies from 04.30 and 10.00 Monday to Friday. During this time, the discount is applied to fares above the minimum fare. This minimum fare does not apply to:
4.5.1. Advance tickets; or
4.5.2. journeys on public holidays; or
4.5.3. journeys during July and August.
At the time of printing, the minimum fare is £12. The minimum fare is subject to change during the validity of your Railcard - check website for the most up to date information.
My reading of that is actually that, given your fare was £17, applying the 1/3rd discount correctly takes it down to £11.30 (rounded) regardless of ticket type.
If the net effect is to have all fares £12+ when a Railcard is applied during that period, then the wording ought to be something like, "During this time, a fare which would fall below £12 after applying the discount shall have a fare of £12.", or something like that.
The current term could be interpreted to
gross fares, whereas they really ought to reflect what should happen if a
net fare is below the minimum.
But that is frankly challenging the interpretation of a clause on a Railcard which has existed for a
very very long time. I don't have deep enough pockets to start a 'group claim' on this!
I otherwise agree with
@Watershed - that's what you've been told as a consumer, you're entitled to rely on it, but doesn't mean you won't get a hard time somewhere along the way.