Quick Question on Current Event

just make sure to buy an arcades, crowds or bowling alleys and anything left in the mystery box before it disappears

All it is is a final reminder that the event is coming to an end and make sure you’ve done everything and collected limited time items before the event ends.

I think that it's something that EA have added to the game as a warning that the event is about to expire and to purchase what you want and try to finish as soon as possible as in the past there has been multiple complaints about the event suddenly ending and players having forgotten to purchase certain items from the event.

It's done it for me to I've been done for like 5 days you have all the stuff so you completed it

I think there is 3 messages that pop up at the end of every event and after the event ends including this one. The first one appears about 3 days before the end date saying something like the event is ending, finish up before the event ends. The second one says there is 24 or less hours left of the event. The third one says the event has ended and you can finish up the current prize you're working on after the end date which usually appears shortly after the event has ended.

Those messages only popped up for me when I haven't got all the prizes when the event is nearly over but like you the messages appear in the current event even though I unlocked all the prizes a few days ago.

Another thing I found is that the new premium characters (Cosine Tangent, Strawberry and Wall E. Weasel) in the event's store still says "Earns Event Currency" above the amount of donuts they cost which usually disappears when I get all the prizes in the event or the current act.

I had got all prizes, the very last one is a decoration of a computer, (can't remember exact name), but I am still doing tasks for Cleric Marge and Lady Millhouse, and I have had the messages as well.
I just put it down to automated system notification that it is ending soon and to get anything you want from the store.