Note:
The issue is reproducible only in dev mode.

Steps to reproduce:

1. Configure custom tag (see below)
2. Create content, insert a custom tag in the RichText field
3. In the custom tag insert:
   1. First paragraph with text
   2. Second paragraph with URL
   3. Third paragraph with URL
4. Publish

Result:
You should see an Symfony's exception page (see attachment) with message:

An exception has been thrown during the rendering of a template ("Warning: DOMXPath::query(): Undefined namespace prefix") in "@admin/fieldtypes/preview/content_fields.html.twig".

Expected result:
Custom tag should be rendered normally.

Configure custom tag:

1. In ezplatform.yml add:

   ezpublish:
       system:
           default:
               fieldtypes:
                   ezrichtext:
                       custom_tags: [bulbo]
   

2. Moreover, in ezplatform.yml or config.yml add:

   ezrichtext:
       custom_tags:
           bulbo:
               template: 'AppBundle:custom_tags:bulbo.html.twig'
               attributes:
                   title:
                       type: 'string'
                       required: false
                       default_value: ''
   

3. In your AppBundle, create the following custom tag template (in AppBundle/Resources/views/custom_tags):

   <div class="bulbo">
       {{ content|raw }}
   </div>
   

4. Clear the cache.

More information:
The XML is converted first by EzSystems\EzPlatformRichText\eZ\RichText\Converter\Link then, the next converter is EzSystems\EzPlatformRichText\eZ\RichText\Converter\Render\Template which does getCustomTemplateContent -> that “extracts” `<para>` from section and calls EzSystems\EzPlatformRichText\eZ\RichText\Converter\Aggregate::convert again. Therefore, Link converter is used again, but this time without <section> in XML, just only separated, multiple <para>. Each of them has own namespace declaration(s) and for the first (without link) xmlns:xlink is missing.