It is hard to say what the cause is without specific details, but we have a number of articles related to this sort of behavior, I suggest starting with the ones below.
Also note that depending on your specific infrastructure that a device between the client and Qlik Sense could also be responsible for the session ending before the time set in Qlik Sense.
I know this answer is not very specific but there are a number of factors and permutations of this behavior.
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