When shocking your water it's best practice to always remove the Sutro monitor (and everything else) from the water for 24 hrs or until sanitizer levels return to normal.
Shock levels of sanitizer could alter the appearance of the Sutro monitor.
Keep in mind, the Sutro monitor measures up to 5 ppm Cl2. In order to measure high levels of sanitizer, use a test kit containing the FAS-DPD method or test strip that measures higher sanitizer levels.
If using a normal dose of non-chlorine shock (Monopersulfate aka MPS) it is ok to leave the monitor in the water but know that your chlorine test results may be skewed if the MPS hasn't fully dissipated. If you are double shocking then we recommend removing the monitor prior to adding MPS.