ProtoTruth's #ignoringFields method accepts integer field numbers, so supplying field numbers from the wrong protocol buffers is possible. For example:

message Bar {
  optional string name = 1;
}
message Foo {
  optional string name = 1;
  optional Bar bar = 2;
}

void assertOnFoo(Foo foo) {
  assertThat(foo).ignoringFields(Bar.NAME_FIELD_NUMBER).isEqualTo(...);
}

This will ignore the Foo#name field rather than Bar#name. The field number can be turned into a Descriptor object to resolve the correct nested field to ignore:

void assertOnFoo(Foo foo) {
  assertThat(foo)
      .ignoringFieldDescriptors(
          Bar.getDescriptor().findFieldByNumber(Bar.NAME_FIELD_NUMBER))
      .isEqualTo(...);
}