WSH 5.7's shell.Run executes wrong Scripting Host engine?

Question

I use a WSC (a COM server written in JScript and run with scrobj.dll) that starts also other script files using shell.Run("TheOtherScript.js");

The WSC server is instantiated with new ActiveXObject(PROGID). Until WSH 5.6 I could set the execution engine for shell.Run with the command

WScript //H:CScript

or with //H:WScript to get the engine I need. For interactive mode I need WScript that pops up a message box on WScript.Echo; for non-interactive mode that is prohibited, since it holds the execution.

After update to WSH 5.7 the WScript.Echo in TheOtherScript.js always causes a MessageBox, independend of the //H: parameter I passed before. This breaks all unattended script execution.

I think this is a bug in WSH 5.7, when can it be fixed, how can I build a workaround?