What filter are you referring to?
salesforce_w2l_lead_source but it seems you’ve changed your question midstream, so that is no longer relevant.
It is not salesforce issue, the data is still mapped from wordpress.
Are you sure? What is the field’s API name in Salesforce? I don’t see that as a mappable field in the web to lead form generator, so it does not appear to be a field you can set via the web to lead API.
Apologies for the confusion, it is “Lead Source Detail” not “Lead Source”. I understand “Lead Source” field is mapped in the form and can be easily edited.
Lead Source Detail is not a field Web to Lead supports…
The fields supported are listed in the FAQ. Custom fields are also supported.
Fields in Salesforce that are not supported by the web to lead API… is something you’d need to take up with Salesforce (or use the REST API – but that’s another plugin).