The reason why that happens is technically a DoW/DoM/DoH classes don't have GP, therefor it is registered as 0 meaning when you switch classes to a Gathering class from anything but a gathering class you will have their defult 0GP after switching.
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Isn't it obvious? Because reasons...
All in all, I was upset at first about it...but I have since learned to try & plan ahead in all things that I do & it rarely bothers me. Only when I get an "out of the blue" invite to go tank BCt1 & I have to wait the full 7 minutes for hallowed ground to be ready even when I have been crafting for 2 hours...& other similar situations. The whole GP thing is a little annoying...but it's only about a 5 min or so wait for full GP so I just change jobs about 2 game time hours before an unspoiled node.
Being able to switch to a combat class when attacked on a gathering job would make no sense as they gave those classes "Stealth" for a reason.