SSRS parameter value from calling report gets replaced by default value on changing filters

Hello,

we are using a report server which runs our reports (2012 version).
There is a report A which calls report B and passes a datetime parameter.
The datetime parameter is a hidden parameter which has a default value =Now().

On calling report B from report A the datetime parameter gets passed correctly.

Now if I change a visible parameter from report B and press "Show report" Button to render the report again, the date gets reset to its default value.

Please let me know if this is desired behaviour, a bug or a wrong setting on my accord.

Thanks in advance,
Christian