My sentinel will hold a negative when the ADV slider is shut but not when the slider is open.

Possibly the servo in the Poseidon first stage is leaking gas when in negative pressure, if a in-line BOV shut off is fitted close it and try the negative test.
If a in line BOV is not fitted then remove the BOV hose and block with thumb, then try test again. If the negative test passes then a slightly leaking Poseidon servo valve is at fault.
Due to the design of the Sentinel this issue will not effect the safe operation of the rebreather, but you may consider either fitting an in-line BOV shutoff or replacing the servo on the Poseidon.
If an in-line BOV shut off is fitted once the negative test is completed remember to open it.

Further Resources

Assembly & Testing Document