Permanent Voltage Drop

I understand that the battery degrades over time.

However, without any apparent reason I found my scooter does not charge above 51.6V while a full charge used to be 55.9V. (I own a Zero 9).

It hasn't been used for two weeks before this happened, but I left it roughly half-charged, which should be ideal for a long-term idle time.

Any clues on what's going on?