That suggests that the 303 is detecting an undervoltage on the 1V supply input when the xcore is connected. does this happen with a dummy load? (The 300ma dummy load I suggested is absolute max as stated in the L1 design docs, you could use a smaller load of course.)I am not sure, I will check. I am thinking it is 1v considering it is used as the input.
EDIT: Nvermind, it looks just like TRST_N and RST_N, it is pulled to 3v3, drops low, then jumps back up. But this is also staying low when the core is connected.
Also the design examples from Xmos do not use a delay capacitor on the CD pin, not sure this would have an effect but you could also try disconnecting it.