Sudden battery drop, 70%-->0%, on newly replaced battery

I have replaced my battery with what should be an original battery for the Huawei P8 GRA-L09, battery nr: HB3447A9EBW.

After having assembled the phone again I experience some weird issues with the battery level dropping from 50%, 60%, 70% or even 100% in an instant. I have tried calibrating the battery by letting it die completely, recharge to 100%, let it drain completely again and recharge to 100%. I have tried different battery calibration apps and a factory reset - nothing helps.

I contacted the company I bought the battery from and got a replacement - still same issue.

Unfortunately i damaged the original battery when taking it out, so i cant use that as a reference.

All stats seems to be similar on the new batteries as on the old one, but for some reason the width of the new battery is a few millimeters smaller than the old one, meaning that it does not fill out the space completely. Has anyone else experienced the same?

Any suggestions on what my problem might be will be most appreciated!