MCP73831: VBat Voltage drop with new uncharged batteries
This is most likely self-discharging of the defective battery, or - hopefully - a feature of the charging IC to prevent the battery from catching fire. LiPo batteries should be disposed of if they're discharged below 3 volts or so because they get damaged when they're over-discharged.

Don't continue using this battery. It is a fire hazard. Get a new one.

Also, don't throw the battery in the trash - put it into a battery recycling container.