In my experience, the DOSS Soundbox-Pro is a very fastidious device.
You should keep almost charged if it is underrun the power cannot charge up. The charging red LED flashes continuously without charging. When charging properly the red LED just lit.
If you change both the cable and the USB charger should solve your problem. I found a great combination of charger and cable, an old OnePlus One charger and cable works well.
I guess it could be a firmware error because I have two, and both behave the same.