Unexpected `type.arguments.not.inferred` error

Test case (reduced from other code):

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.List;
import java.util.Optional;
import java.util.stream.Collectors;
import java.util.stream.Stream;

class Walker {
  List<Foo> b(Path c, ClassProvider classProvider) {
    try (Stream<Path> walk = Files.walk(c)) {
      return walk.flatMap(
              g -> {
                return StreamUtils.optionalToStream(classProvider.provide());
              })
          .collect(Collectors.toList());
    } catch (IOException e) {
      throw new IllegalArgumentException();
    }
  }

  abstract static class ClassProvider {
    abstract Optional<? extends Foo> provide();
  }

  static class StreamUtils {
    static <M> Stream<M> optionalToStream(Optional<M> n) {
      return null;
    }
  }

  static class Foo {}
}

If I run it through the Tainting Checker, I get these errors:

$ checker-framework/checker/bin/javac -processor tainting Walker.java
Walker.java:14: error: [return] incompatible types in return.
                return StreamUtils.optionalToStream(classProvider.provide());
                                                   ^
  type of expression: @Tainted Stream<@Tainted Foo>
  method return type: @Tainted Stream<? extends capture#01 extends @Tainted Foo>
Walker.java:14: error: [type.arguments.not.inferred] Could not infer type arguments for StreamUtils.optionalToStream
                return StreamUtils.optionalToStream(classProvider.provide());
                                                   ^
  unsatisfiable constraint: @Tainted Foo <: capture#01 extends @Tainted Foo
2 errors

I get similar errors with the Called Methods Checker / Resource Leak Checker, but not with the Nullness Checker (I do get the expected nullness error in optionalToStream).

Are these errors expected?