I really don't know, my thought would be either leeching of the plastic to the juice causing a flavor change - or despite washing, some trace amounts of left over flavor/odor from the previous juice interacting with the current juice you're vaping. For instance in plastic, maybe vaping coconut. Wash it all out, soap and water and all is well (hopefully). Now vaping pineapple in the same washed tank yet getting either a muted or off taste.
Going back to the thoughts on storing food in another container like reusing a plastic container that had onion dip in it (or anything strong flavored). The dip tub is empty, washed with soap and water, now putting jello in it to set up. Jello is sensitive to flavor contamination. Go to eat the jello and it tastes a bit off, but why? The container was washed but not all the odor of the previous contents were able to be removed, slight cross contamination of the onion odor with the jello.
In other words the inability to ever get plastic truly clean no matter how much it's washed. Obviously food containers made of plastic like chip dips come in, cottage cheese whatever isn't polycarb so it's not a totally accurate comparison. Glass on the other hand doesn't retain those odors. It can be washed, cleaned with alcohol, sterilized so nothing is left behind. No cross contamination or muddying of multiple flavors/odors.
Some people won't drink beverages that come in cans (soda, beer etc). Some don't like the flavor of soda in plastic either and prefer glass bottles. It may also depend on how the tank is set up, for instance on my poly clearomizers there's a soft rubbery seal that runs on top of the chimney, helps seal the tank when the drip tip is screwed down tight. Whatever material that is it retains flavor like mad. No washing can help it. I vaped a peach flavor juice, washed it for 10min with soapy warm water. Repeatedly. Everything after that had a funky peach aftertaste because of that silicone rubbery seal material that picked up a flavor it'd never let go.
Totally guessing because I'm no scientist so spitballing in the dark.