ltc6803-1 self test and clear in STCVAD command

hello ADI,

i'm currently testing LTC6803-1 with Arduino, i'm having 2 chips in daisy chain, each chip is connected with 4 x 3.6v battery.

For software i'm using the LTC Linduino example code, and i'm trying to extend it a little bit with other functions.

i'm having a weird result when i'm sending clear with 0x1D 0x93 directly when the chip is powered on, voltage register readout as follows(value in hex)

 IC 1 C1:AA, C2:AA, C3:AA, C4:AA, C5:AA, C6:AA, C7:AA, C8:AA, C9:AA, C10:AA, C11:AA, C12:AA,
 IC 2 C1:AA, C2:AA, C3:AA, C4:AA, C5:AA, C6:BD, C7:AA, C8:AA, C9:AA, C10:AA, C11:AA, C12:AA,

and the response PEC code is calculated correct! but the result is very strange. not 0xFF as i expected.

so i ran write config of 0x71 to CFGR0 to set CDC to 1, and all other control register to 0x00, then immediately clear again, i got the 255...almost

 IC 1 C1:FF, C2:FF, C3:FF, C4:FF, C5:FF, C6:FF, C7:FF, C8:FF, C9:FF, C10:FF, C11:FF, C12:FF,
 IC 2 C1:FF, C2:FF, C3:FF, C4:FF, C5:FF, C6:2E, C7:FF, C8:FF, C9:FF, C10:FF, C11:FF, C12:FF,

the C6 on IC2(upper, top) is always 0x2E, and if i ran a self test with 0x1F, 0x9D afterward, i got

 IC 1 C1:AA, C2:AA, C3:AA, C4:AA, C5:AA, C6:AA, C7:AA, C8:AA, C9:AA, C10:AA, C11:AA, C12:AA,
 IC 2 C1:AA, C2:AA, C3:AA, C4:AA, C5:AA, C6:BD, C7:AA, C8:AA, C9:AA, C10:AA, C11:AA, C12:AA,

again the C6 on IC2 is not 0xAA as it should.

the pcb is a custom made based on reference design, and the cell voltage reading and some other functions are working perfectly.

if a swap the 2 board(bottom to top, top to bottom) i'm still getting the IC2 C6 == 0xBD after self test with 0x1F, 0x9D so i don't think its a hardware problem.

if i use self test 0x1E 0x9A, i'm getting IC2 C6 = 0x0F

 IC 1 C1:55, C2:55, C3:55, C4:55, C5:55, C6:55, C7:55, C8:55, C9:55, C10:55, C11:55, C12:55,
 IC 2 C1:55, C2:55, C3:55, C4:55, C5:55, C6:F, C7:55, C8:55, C9:55, C10:55, C11:55, C12:55,

so my question, 

(1) is there any pre-conditions for clear command (0x1D 0x93)? for example start CDC perhaps?

(2) why cell 6 on higher IC is not set to 0x55 or 0xAA and how to troubleshoot?

thanks in advance.