How did you obtain it?
The thing I'm thinking is that the battery diagnostic system was corrupted, and/or perhaps someone "reflashed" the data that contains things such as the estimated battery capacity and the running cycle total.
At that age it's likely that it needs a new battery. I'm not sure if it would be eligible for an OEM battery from Apple though if it's been modified. However, it may still be eligible for a battery replacement service if the only issue is the battery.
A 2015 MacBook is considered "vintage" (up to 7 years since last distributed) by Apple, although Apple may have Mac batteries available for up to 10 year from the last time of distribution.
Obtaining service for your Apple product after an expired warranty - Apple Support
It seems that the battery replacement on a 12" MacBook is really difficult and costs about the same as the newer MacBook Pro batteries. Apple does this as a complete top case replacement when done by Apple, as the battery is glued into place. It's also supposed to be a real pain to do. What Apple will do is replace the entire case, and then refurbish it with a new battery at a central location to put back into service for another customer.
