Just throwing my hat in the ring, I’m having the same problem but not actually going to blame iFixit in this case - more on that in a sec.
Both a friend and I have/had Mid 2014 15-inch MBPs that we bought at the same time. Maybe about half a year ago he decided to swap out his battery with an iFixit replacement and had the exact same issue: battery went from about 60% to 7% instantly and coconutBattery showed his battery health to be in the low 80s after just a few months. This was on a battery he got as a replacement because his first battery (also from iFixit) was doing the same thing. He ended up just selling his MBP back to Apple as credit for a new one (I guess they didn’t run any diagnostics or didn’t care because they did accept it, though he only got a couple hundred bucks).
Fast-forward to now and I just got a replacement battery from iFixit and of course am experiencing the exact same issue, except mine drops to 7% when I hit around 35% battery. After just four charge cycles my battery health dropped from 100% to 96%. Just to be clear - I made sure to calibrate the battery exactly how iFixit recommends, and I did all the usual stuff like reset SMC/PRAM, etc that never actually seems to work (but did it any way).
Furthermore, I have never used a 3rd party charger, I did buy a second charger to keep in my office a couple years ago, but I bought it directly from Apple. I’ve also absolutely babied this machine, I’ve never dropped it and it still looks brand new to this day. The only issue that shows up when I run a diagnostic is that it says the battery needs service (despite being a new battery) though it says that it’s still safe to use the machine.
I’m not the tinfoil hat type, but I’m going to wear one in this instance. We are WAY beyond coincidence here. Two batteries from a friend, one from me, all having the same exact problem. Users here have reported replacing their battery as many as SIX times. This is also not just a “bad batch” because this issue really started happening about two years ago from what I can tell (heck this very thread is now two years old) and bad batches don’t last for two years.
I’m really starting to wonder if Apple themselves are up to something. It’s just weird that all of this started really happening at around the same time and it’s still happening to this day. Apple is fighting hard against right to repair, something that iFixit is very much for (and is largely the face of the whole movement). I absolutely wouldn’t put it past Apple at this point to pull something shady because the more people see that iFixit batteries constantly fail the more they’ll think “well, Apple is right - third party replacements STINK!”
I’ve never had any issue with any other iFixit battery and I’ve replaced batteries on multiple Kindles and such. They’ve always exceeded expectations. Of course different batteries will come from different sources, but it’s hard for me to believe that iFixit, who has a great reputation otherwise, would not have addressed this issue by now.
Other people have reported luck with OWC batteries so I just ordered one from there and we’ll see how that goes. I super promise I’ll update this thread with my findings (I set a reminder so I don’t forget, I know people always say they will update threads and never do - I will).