Both you and whoever told you that at Carnival are incorrect. I have achieved 3 milestone cruises and am working toward the free cruise. If you did not receive the card on your 25th (revenue) cruise, then Carnival messed up. Shocking.
https://help.carnival.com/app/answers/detail/a_id/1173/~/guest-recognition---milestone-rewards-program
Milestone Rewards Program denotes a guest’s 25th, 50th 75th and 100th revenue cruise with Carnival. Guests will receive a special Milestone Sail & Sign card on their corresponding Milestone cruise. We want to distinguish guests on their Milestone sailing with a unique card so that we can make his/her Milestone experience extra special and memorable. After this milestone cruise, they will revert to the card that corresponds to their VIFP level.
In appreciation for their loyalty, the guest will also be granted the opportunity to redeem a future non-refundable onboard credit equivalent to 25%, 50% or 75% of the cruise ticket price, corresponding to the milestone reached. And when a guest reaches his/her 100th cruise, will qualify for a FREE Caribbean, Mexican Riviera, Alaska or Australia cruise of their choice (8 days or less, for 2 guests), in accommodations up to and including Ocean Suites (category OS/OT). This onboard credit may be applied to any future cruise in accordance with the terms and conditions below.