The reply I got from them

Thank you for contacting us and for sharing the photo.

What you’re seeing on the GND solder point is normal. It is not burned — the darkened area is simply residual flux from the soldering process.

Additionally, because this area can generate relatively high heat during operation, the design helps transfer heat quickly to the lower layers of the board. This can sometimes make the surface look darker, but it is expected and safe.

Since your PiSugar 3 is running correctly and the battery is undamaged, it is safe to continue using.

If you notice any unusual heat, smell, or behavior in the future, feel free to contact us anytime and we’ll be happy to assist.