After my tile disappeared when the Christmas update came down, I immediately built another (up to 3.5 days now) and just left it floating there once it had completed. A couple of my neighbours have tapped on it, but I am not particularly looking for that - it just gives them a choice to go for 1000XP rather than just 10 gift cards for one of their taps in my town.
Seems to work just the same now as it did before. I imagine some people just aren't interested in continuing to build SP while the Christmas event is on, and others have possibly reached the maximum SP tiles and aren't allowed to build any more.
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