My guess is that it was designed that way as a side effect of not being able to use a cordial while at a gathering node; being in cast stance is treated the same way as having a node open. Back in ARR, there was no reason for a fisher to use GP, so nobody noticed/cared.

I would also guess that it's been kept that way through HW because of Patience (II) and Fish Eyes, which wear off after you leave casting stance, just like miner/botanist buffs wear off when you exit a node.