I'm using a system plugin that adds form fields to the attribs field of articles.
It's loading an xml file with a nested fields group (subgroup of attribs). <fields name="newslinks4irc">

<?xml version="1.0" encoding="utf-8"?>
<form>
<fields name="attribs">

 <fieldset name="newslinks4irc">

  <fields name="newslinks4irc">

---Several fields here---

  </fields>

 </fieldset><!--/newslinks4irc-->

</fields>

</form>

The fields are using showon. E.g. showon="linktype:url"

...
   <field name="linktype" type="list" default="url"
   label="Link type">
    <option value="menuitem">menuitem</option>
    <option value="article">article</option>
    <option value="url">url</option>
    <option value="JNONE">JNONE</option>
   </field>
   
   <field name="url" type="url" validate="url"
    description=""
    label="Enter url"
    showon="linktype:url" />
...

This worked fine with Joomla version 3.7.5

Since Joomla 3.8.0 showon doesn't function anymore and all fields keep hidden.

• You can download a demo plugin here: [URL removed by ReLater] It's reduced to the relevant parts.
• Install it and activate it.
• Open an article in back-end.
• You'll see an tabulator "Demo Plugin contentformaddfieldsghsvs".
• Change settings of option "Link type".
• Expected: Other fields show and hide depending on showon attribute.
• Actual: They don't with Joomla 3.8.0

Test it with Joomla 3.7.5. Then update Joomla to 3.8.0 and test again.

Joomla <= 3.7.5 (tested with several sites and a new installation)

Joomla 3.8.0 and current staging (tested with several sites and a new installation)