SSRS Linked Report with allow multiple parameters
I' pretty sure you actually have to use a function for that.

From a quick google search, here are a few options:

=SPLIT(JOIN(Parameters!&lt;your param name&gt;.Value,","),",")


Or

=join(Parameters!&lt;your param name&gt;.Value,",")


One thing I remember vividly from my SSRS/ReportBuilder days was that when you're passing the value in from the main to the sub report (and it has multiple values), you pass the param in as "[@paramName]" and not "Parameters!.param.Value"