Custom tags starting with embed are removed even when added to ACF

[jswiderski]

Are you reporting a feature request or a bug?

Bug

Check if the issue is already reported

Provide detailed reproduction steps (if any)

1. Use below configuration:

CKEDITOR.dtd['embedded-expression'] = { '#' : 1, 'span' : 1, 'img' : 1 };
CKEDITOR.dtd.$inline['embedded-expression'] = 1;
CKEDITOR.dtd.body['embedded-expression'] = 1; 

var editor = CKEDITOR.replace( 'editor1', {
    extraAllowedContent : 'embedded-expression[*]{*}(*)'
});

2. Load sample page in browser and paste the following HTML in source mode <embedded-expression data-test="test">test</embedded-expression>
3. Switch to wysiwyg and back to source.

Expected result

Tags stays

Actual result

Tag gets removed.

Other details

• Browser: Any
• OS: Any
• CKEditor version: N/A
• Installed CKEditor plugins: N/A

It seems these lines are responsible for that result: https://github.com/ckeditor/ckeditor-dev/blob/a0dd80a/core/htmldataprocessor.js#L802-L806.

A simple workaround would be changing tag name so that it doesn't start with embed.