Cascading data subscription options

I am trying to set up a data driven subscription and it works great except when 2 parameters are set to accept dynamic values ​​instead of static ones.

The string is basically

Parameter1 -> read from
Param2 database -
> read from Param3s database - > read from
Param3 database - > split Param3s (multi-valued parameter, valid values ​​depend on Parameters 1 and 2)

Note that Param3 is a multi-valued parameter and therefore its value is populated by the proxy from a string parameter (param3s) that is stripped.

The above doesn't work and gives the following error when saving the subscription

This report requires a default or custom value for the "Param3" report parameter. To run or subscribe to this report, you must provide a parameter value. (RsReportParameterValueNotSet)

However, when setting the parameters Param1 and Param2 to static values, it works.

There is nothing useful in the ReportServer logs to help determine what is going on.

Does anyone have a key?