Glad to hear that it was a simple mistake (not pre-filling the carto) - I'm guessing your tank works perfectly now you are prefilling! I have never had a single issue with my tank when I do this.
If a carto starts to taste burnt, it's either dead or dying. Better to toss it and use a new one. Luckily they are cheap!
I made exactly the same mistake when I first got a carto tank system, so you are not alone
I buy the pre-punched single coils. (They usually have 3 holes) .. On ones that aren't pre-punched I generally only put one hole in mine. Again, never had any issues with one hole. Even with the thickest juices.