A dialog appeared telling us that only 7/8 people are in instance so only 1 chest will appear.
[Yes] - Accept that you get only 1 chest
[No] - Closes dialog but nothing happens, timer continues ticking.

Since the tome is given on Boss death, our 2 chest weekly party became a 1 chest just because someone dced and is not able to come back.

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The only viable workaround is to wipe the instant someone disconnects.