“The arena page says it has had one day left until completion for three days now. Is this a bug?”This is not a bug. When you expand your arena, it gives you an estimated time (for example 8 days). This does not mean that exactly 8 days later your arena will be built. It is an estimate, so it could be built after 6 days, or it could be built after 8 days exactly, or maybe the workers are a bit lazy and it takes 11 days to build. Generally, the estimate time tends to be on the optimistic side, so if it says 6 days to complete, it will probably take longer than that.
estimation cannot indicate a delay higher than 15 days, altough the real duration will be higher than 15 days.