The way that I've done this successfully in the past was to use the expressions to replace the Excel Filename property with the filename from some variable.

My observation has been that the connection string property for a spreadsheet does not ever contain a filename, even when using a static filename. (but that is not to say that SSIS does not add the filename property to the connection string at runtime to produce it.

At least that how i've done it in a ForEach container and it has always worked. Never had to do anything with the connect string property, only the filename property.