So much potential waiting to be unlocked. No drippers or KFL/Fogger or similar? You're not getting anywhere close to the potential of these juices with the Evod/PT/T3D tanks.
Moving along.. I'm guessing you're also rinsing out the tanks and bases when changing flavours? I can't say that I experienced flavour creep when I used to use the Protanks, but it's possible that there may be some being held on in the nooks and crannies in the tanks. I would expect that most juices would right away take over a wee bit of leftover flavour, but if you were going to a much lighter juice compared to a stronger previous one, there could be some residual flavour left over.
The air tube and drip tip may also hold on to some residual flavour although that should not last long either.