[electroken]

We have also noticed that the ADXL345 functions correctly with the flat cable but not the round cable.

The ADXL345 uses a I2C serial interface. Pull up the I2C specification from NXP Semiconductor (they invented it) and you'll find a maximum allowable bus capacitance. I'm working from memory here, but IIRC the maximum is 400pF, which is not a lot.

In the flat cable the I2C signals (SDA and SCL) can be next to two other signals at most. In the round cable they're tightly bundled with 35 of their friends and I suspect the bus capacitance is much higher.

I like explainable things, and that's the best I've got for this.