ImageColumn / DefaultImage not showing

Is there anything I am doing wrong? The default image does not show up:

ImageColumn::make('thumbnail_image_path')
->visibility('public')
->label('Cover')
->toggleable()
->imageHeight(100)
->disk('public')
->defaultImageUrl(url('images/cover_placeholder.png')),

Default image html is only this:
<p class="fi-ta-placeholder"></p>

Or is this a bug? Thx in advance.