The client side encryption spec is a little vague/misleading about whether an error should be raised for automatic decryption (i.e. when bypassAutomaticEncryption is set).

It says:

Drivers MUST raise an error when attempting to auto encrypt a command if the maxWireVersion is less than 8. The error message MUST contain "Auto-encryption requires a minimum MongoDB version of 4.2".

But incorrectly indicates that an error is raised on auto decryption against a pre 4.2 server:

However, errors are only raised for automatic encryption/decryption against older servers

We can also add a spec test to validate that automatic decryption works on pre 4.2 servers.