Accessing an array out of bounds is undefined behavior so the compiler can assume that it never happens in the LHS of the && expression. It is then jumping through hoops (optimizations) to notice that since ARRAY_LENGTH is the length of the array, the RHS condition must necessarily hold true (otherwise UB would ensue in the LHS). Hence the result you see.
The correct check would be idx < ARRAY_LENGTH && g_ptrArray[idx] != nullptr. This would avoid any possibility of undefined behavior on the RHS since the LHS has to be evaluated first, and the RHS is not evaluated unless the LHS is true (in C and C++ the && operator is guaranteed to behave this way).
Even potential undefined behavior can do nasty things like that!