I created a pull request, so copying the text from that:

If we have an interface like:

public interface SampleInterface {
 int getFirst();
 String getSecond();
}

which is implemented as:

public abstract class SampleImplementor implements SampleInterface {
 public abstract boolean isThird();
}

and has a concrete implementation that's called `SampleImplementorImpl`.
When `PojoBuilderHelper` goes to create the property models, it only checks
for methods on the current class and super classes - not interfaces. In
the above example, this means the property model will only have "third" entry -
no "first" or "second" property model. Today, you can manually get around that by
creating a @BsonCreator by hand:

public abstract class SampleImplementor implements SampleInterface {
 @BsonCreator
 public static SampleImplementor newInstance(
 @BsonProperty("first") int first,
 @BsonProperty("second") String second,
 @BsonProperty("third") boolean third) {
 return new SampleImplementorImpl(first, second, third);
 }

public abstract boolean isThird();
}

The presence of the `@BsonProperty` on the `@BsonCreator` method will
create the property models. Conversely though, if you want to leverage a
`Convention` implementation to dynamically create a `InstanceCreator`
that knows how to find `SampleImplementorImpl` above, you won't be able
to. `InstanceCreator` is only provided properties for which
`PropertyModel` exists, so above, since `PojoBuilderHelper` didn't
discover the interface fields, and there is no exposed API to add
property models, your `InstanceCreator` will never be provided the
`first` and `second` fields present on the interface.

Simply put, if you provide the pojo codec a class that is not concrete,
extends an interface for methods, and does not have a `@BsonCreator`
annotation, there is no way to implement a `InstanceCreator`
implementation that works for the non concrete class. You get stuck in
a place where the class can serialize since the concrete implementation
`SampleImplementorImpl` is provided at runtime, but then you have no way
to deserialize it since usages in the code only reference `SampleImplementor`.

We've worked around this problem for years and created 700+ hand written
`@BsonCreator` annotations, so at this point I want to fix actual the problem.

This fix is relatively straight forward: Update `PojoBuilderHelper` to
scan implementing classes and interfaces, which provides a fully
populated property model.