it looks like showon is not working for a global option. it works correctly is changed from global to a local setting

Issue confirmed.

System information

3.8.2-dev
Multilanguage Site
macOS Sierra, 10.12.6
Firefox 56 (64-bit)

MAMP 4.1.1

• PHP 7.0.15
• MySQLi 5.6.35

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  _{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/18423.}

Concretely, it will only work for single contact menu item or when editing a contact IF the global settings Contact Information is set to Hide.
In that case, switching to Show in the menu item or the contact itself will work as intended and display the fields.

True, @infograf768. So the Issue is that Global Setting "Contact Information: Show" isn't respected in single Contact and Single Contact-Menu.

The problem is similar for articles.
Global Options display Linked Titles if Show Title is set to SHOW.

The showon is missing in article xml (should be added btw), but if I add a showon to obtain

			<field
				name="link_titles"
				type="list"
				label="JGLOBAL_LINKED_TITLES_LABEL"
				description="JGLOBAL_LINKED_TITLES_DESC"
				class="chzn-color"
				showon="show_title:1"
				useglobal="true"
				>
				<option	value="0">JNO</option>
				<option value="1">JYES</option>
			</field>

When editing an article Show Title would display Use Global (Show)
and Linked Titles field will not display until Show Title is set to Show.

Therefore the issue is not specific to com_contacts.

Regarding this I don´t know if related but

I had "sliders" as global configuration and "plain" as contact configuration. The contact form appeared as "sliders"

@irbian can you please open a new Issue?

Sure

@AlexRed Is this issue still existent in latest J3?

4 and half years later I think that can be closed