After reading this thread, I went to the Marriott Rewards site and logged in, and lo and behold, MY MegaBonus offer has now changed too!

It had been 15 nights for 35k bonus.
Now it shows 7 nights for 15k bonus.

No bonus has yet been credited, and the 15th night is being completed tonight, so I am at least hopeful that the correct bonus will be applied (otherwise, I assume I'd have gotten the bonus already.)

Definitely need to keep an eye on this one.