That is because you are doing it wrong.
Watch the mana tick up. Then wait for the mana to tick down 1 mana. When that 1 mana change occurs, that was the "real" tick and you can cast off that mana. If you try to cast off he visible full meditate tick value, what you described will happen.
It is unknown if this is classic cause we never were able to see our mana pool values back then.
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