The reason why the alarm was not triggered is because the HH alarm value for dewar was set to 90%. This seems logical (or even too low), but the conditions for this alarm are the following:

TLAUX_Pr_15Dewar_AuEHH:= ATLAUX_Pr_YY11503B.OnSt AND ATLAUX_Pr_15PCO.RunOSt;

To trigger the alarm "Dewar is full but not in service" the HH on dewar has to be ACTIVE and the valve position to be open on the side B backup(YY11503B open).

 

So for all of it to work properly, the condition HH shall be modified to "NOT L" (with L set to 20%) for all of it to make sense.