Per Rascalmom these are "rare". Certainly I've not seen them often. However, when I do, they're causing nothing but frustration. Why? Because, of the three I've seen, each one put me up to my imagination limit for shrine donations for the day and I've essentially lost the bonus. The one I just "received" today gave me nothing at all. One earlier in the week gave me a whole 1000 extra.
Frankly speaking, things like this are what make people quit playing. Ie. telling them, "Here's a nifty bonus that doesn't show up except once in a blue moon. Oh, by the way, you can't have it."
So, yes, I consider this a bug. In this one special instance, the daily limit should be ignored and you should actually GET the bonus imagination.