hello,
What you said is correct, in vSphere 55 if the LUN is APD then it has to be marked as PDL if LUN doesnt come back onlinw within next 140 seconds. This will make sure the management service hostd is not keep trying accessing the storage in a loop and getting into hung state later.
Can you check the value of the advanced system setting "Misc.APDHandlingEnable"? If by any chance if the value of it is non-default, ie 0 then it answers the question. The value should be 0 to function APDHandling.
