I’ve always played it that they lose the IPC’s off the National Production Chart immediately (Germany isn’t affected, only Allied Country’s production goes down) but there is no losing of IPCs immediately from the hand. On the Allied Nation’s next turn they would receive the lowered amount of IPCs, unless they first retake the convoy zone.
At least that’s been my take on it–someone wiser on this subject may have a better answer.
Pages 16-17 I think can be a bit confusing on the issue, yes, but I think the example on 17 helps. It mentions income as compared to IPCs. Income would refer to the amount to be received rather than the amount in a player’s hand.
The German player moves four naval units during Germany’s turn.
A German sub in the Central Atlantic moves into the United
States’ Convoy center. Another German sub in the Azores Sea
moves into a British Convoy Center and back out to its original
zone. A German destroyer moves through a British Convoy center
and into the Azores. A German transport in the Central Atlantic
moves through a Convoy center and into the South Atlantic.
However, because a transport has no attack value, it does not
take control of the Convoy center. The German player places
three German markers on the Convoy centers it now controls,
and the IPC chart is adjusted. The U.S. loses 4 IPCs in income,
while Great Britain loses 8. Germany’s income is not affected.
On the U.S. turn, the American player moves a destroyer from
the U.S. Coast sea zone through the Mid-Atlantic to the Convoy
center. Since the German sub is no longer there, the center is
liberated without combat. The German marker is removed and
Britain’s income is raised 5 IPCs.