with the expression "(reg:rcx.u64+reg:rdx.u64),x"

If rcx contains 0x123 and rdx contains 0x100 the result of this expression is evaluated to 0x123.

On commit 9ad7361

This seems to be the problem commit that introduced the bug. cf9d736 . Makes sense, after the first register is read from the register space the "selected_space" is erroneously changed to the "primary_space". So subsequent attempts to read registers will be reading from the wrong space.