You’re not going mad. The ticket should include a link to https://www.nationalrail.co.uk/ticket-types/validity/b1/ on the National Rail website which confirms that it is only not valid between 0430-0929 on weekdays. Please complain to the companies who have told you otherwise.
Also, if you have an Oyster card with your 16-25 railcard attached then the zone 1-6 off-peak cap is just £10.85, and that will be enforced throughout the afternoon peak without you worrying about it. Plus, if Harrow is the furthest out you go (ie zone 5) then the cap reduces to £10.15. It goes down to just £5.90 if all travel is just in zones 1-2.