The search by product type parameter facility uses the mysql function FIND_IN_SET() when searching 'multiple value' parameters. This function requires that the items in the set be separated by commas. The values in the product type parameter fields are separated by ';' and so the function fails. The routines that maintain parameter values must be changed to use commas as the separator when saving and retrieving values from 'multiple value' types. It would also make sense to also change the separator in the 'Possible Values' field to a comma to eliminate any errors that might be caused by having a comma embedded in a value.
As a short term solution (with potential performance penalty compared with a solution involving switching from semi-colon to comma separators) I have implemented the following changed to shop_browse_queries.php:
Line 227 changed from
$comp = "FIND_IN_SET('$get_item_value',`$parameter_name`)";to
$comp = "FIND_IN_SET('$get_item_value',REPLACE(`$parameter_name`,';',','))";and line 235 from