Helo W10M P,
The fact that it happened 4/5 months after you purchased the phone could very well be a defective battery, sometimes that happened, not it the first or second time you charge the battery, but after a while.
Can you explain point nº 2 better: “When the battery level is recovered from 6% to its' original level you just can use the phone for many where it just uses power in a normal gradual way”