BQ34Z100 strange behavior when reading registers

I'm trying to read some data from a battery pack we get already assembled and configured from a supplier.
I know it's a BQ34Z100 inside and amongst others I read the SOC/SOH-Registers from adr 0x02/0x03 and 0x2e/0x2f. Correspondig to the datasheet I could read values from 0 to 100 % like expected. But sometimes the return value equals to 0xFFAB, reading as 65451 unsigned or -85 signed. To exclude a bug in the software, I measured directly on the I²C-lines with a scope and found the same response. Other registers like voltage (0x08/0x09) and current (0x0a/0x0b) are not affected by this behavior.

As this is reproduceable and not described in the datasheet, I'd like to know if this could be an error code from the gas gauge and if so, what does it mean?

Regards Kai