What happened here is you let the cells go too low and they self discharged. Normally when we hit a low voltage cut-off it is under significant load and the cells bounce back to 3.6 volts when the load is cut. In this case the cells slowly discharged, effectively dropping the resting voltage to a level were they would start self discharging.
I am still just speculating but you can be sure those cells will never be the same. Their internal resistance is probably much higher now.
all 5 cells of my other lipo pack are at 3.4v though. Why didnāt that one get crazy unbalanced and low as well?
Also thinking back on it now shouldnāt my BMS have somehow turned my board off by closing a circuit or something even if Iām using a loop key?
Theres no way everybodys boards are killing batteries just by leaving them on are they?
You probably damaged the cells prior to this happening, your pack is really small so it was under heavy use.
Thatās interesting, at which voltage this starts to happen?
I left my mtb switched on for about 28 hours and my 10s 10Ah Turnigy Graphene dropped from 35V to 8.87V total voltage and is puffed and dead now. So pffffff @cryo 1.4V is not bad but I win with 0.89V per cell 
But joking aside Iām planning to use a bms on my new battery, too to save it. But that your cell dropped to 1.4V with bms is strange.
Yea I thought thats what BMS were used for, I think it should have closed off the circuit when it detected cells below 2.9v like it says in the spec sheet but I guess not apparently. Either that or I have a faulty BMS for discharge because I think its highly unlikely Lipos self discharge that much over a couple of days, I remember reading somewhere that they only discharge ~5% a month with no load.
Managed to bring my lipos up back to an acceptable voltage to be charged normally again. Used a benchtop regulated power supply to charge each cell to 3.4v and from there I was able to use my regular charger. No puffed cells or overheating so far, runs just like it did before though I donāt doubt its lifespan was shortened by my fuckup.
Thanks for the insight guys 