You want the caffeine without the sugar, so you grab the sugar-free can — and the aftertaste is the trade-off nobody warned you about.
Sugar does a lot of work in a normal energy drink beyond sweetness — it also masks bitterness and rounds out the flavour. Take it out and the artificial sweeteners used to replace it often leave a metallic or bitter aftertaste that's hard to fully mask.
It's a real formulation challenge, which is part of why so many sugar-free drinks taste noticeably different from their full-sugar versions.
Gum sidesteps this differently — flavour and caffeine aren't trying to solve the same problem in the same liquid. Neuchu is sugar-free with 50mg of natural caffeine per piece. You can find it here.