This is not only an issue of form field type "tag" but also for any multiple form field with class that

extends JFormFieldList

Steps to reproduce the issue

1) In an article add 2 or 3 tags. Save article. Close it.
2) In
/components/com_content/views/article/tmpl/edit.php
after
defined('_JEXEC') or die;
add line
$this->form->setFieldAttribute('tags', 'readonly', 'true');

OR(!)

add a readonly attribute to field definition of "tags" in
/components/com_content/models/forms/article.xml

field name="tags"
type="tag"
label="JTAG"
description="JTAG_DESC"
class="span12"
multiple="true"
readonly="true"

3) Set error reporting in Joomla configuration to maximum.
4) Reopen article of step 1).
5) You'll see a PHP warning
htmlspecialchars() expects parameter 1 to be string, array given in /libraries/joomla/form/fields/list.php on line 66
6) In disabled tags field you'll see your tags.
7) Save article.

Tags field is now empty and tag assignments to this article are removed from database

Additional comments

In /libraries/joomla/form/fields/list.php if readonly is active for a select box a hidden field is created that throws above warning/error because form field e.g. of type tag (extends JFormFieldList) sends $this->value as an array not as a single value.

PR #4261