The approach taken should override the defaults described on Secure Data Connections. We seem unable to reproduce the problem; a value manually set for SslMode is not overwritten by the overwritable default setting of Required.
In the next 20.2.2 release, there will be additional logging to establish what actually happens. I will let you know when available.
In between time, please try to use None instead of Preferred. That completely disables SSL.
As an alternative, could you try InvalidValue instead of Preferred? When applied, it should raise an error like:
itgenasl001:
The keyword ‘sslmode’ could not be included in the connection string. —> System.ArgumentException: Requested value ‘InvalidValue’ was not found…
You can also enable trace logging (when not yet active) and check the DebugView-output for the following string:
Provider ‘…’ meets criteria to handle the login action using (anonymized) connection string ‘…’