I got the same issue. I bought an adapter and a new CO2 tank (filled) at a local keg supply shop. I don't know if the CO2 tank I got has a siphon tube or not (If anyone knows how to tell if a CO2 tank has a siphon tube, without releasing gas and taking out the valve up top... please let me know).
This is the adapter I bought:
https://www.amazon.com/maker-Machine-Adapter-valves-accessories/dp/B0725JG5Q6/
That adapter has the ability to vent the line. I noticed that if I leave CO2 in the line it has that (plastic / propane like) smell. I've read that once the CO2 gas sits in the line it condescends (turns to liquid) and the liquid CO2 is what makes the weird taste... not sure about that tho.
I found that if I leave all the valves on the adapter open it gets better (not as strong but still there)... but then I am losing that CO2 from bleeding the line.. so I don't know if it's worth it in the mean time.
I've also read that if the CO2 cylinder you buy has a siphon tube, it will suck up liquid CO2 from the bottom of the canister and again.. the liquid CO2 is what makes a bad taste.
I've also read that the flavor could be caused by over-carbonation (maybe too many pumps), and the need for some of the CO2 to come out of the water. Again not sure one this one.
Finally, I think some of us are more sensitive to these off flavors than other people. Some of my family do not notice the taste, others do.
Another thing I notice is, if I let the carbonated bottles sit in the fridge for like a day it also isn't as bad (but still present). Thing is, my family goes through like 10+ bottles a day, so it hardly has time to settle.
I got no clue how to resolve this. I've scoured the internet for advice. I've tried to disassemble my adapter (to the best of my ability) and clean it with soap/water and pipe cleaners. I've vented CO2 from the canister and smelled the CO2 (no smell). Right now, i switched back to my almost depleted soda stream bottle (no problems with taste), but that isn't going to last long. Not sure what else to do.
I am sorry to necro a month old post... Was there any resolution? Anyone find anything out that can help?