Adding a field named 'comment' to an entity can not then be displayed in an Entity view, becuase the renderer will find the template named 'comment' (found in core/themes/classy/templates/field/field--comment.html.twig) and use that instead of the template for the widget defined in the view display options.
<?php $fields['comment'] = BaseFieldDefinition::create('string_long') ->setLabel(t('Our comment')) ->setDisplayOptions('view', [ 'weight' => -5, ]) ->setDisplayOptions('form', array( 'weight' => -5, ));
The above field definition results in:
* Add form: a textarea allowing me to type comment and save it - this is correct
* Entity view: when I view the entity using default view builder, the field is ommitted from the view - this is wrong, it should display the value of the entity's 'comment' field.