Default Open Graph metatags are not merged but overridden.

[eirc]

So you got in your layout:

<%= display_meta_tags :site => 'Site Name',
                      :open_graph => { :site_name => 'Site Name' } %>

And in some action:

<%- set_meta_tags :title => 'Page Title',
                  :open_graph  => { :title => 'Page Title' } %>

This results in:

<title>Site Name | Page Title</title>
<meta content="Page Title" property="og:title" />

That is we lost this one:

<meta content="Site Name" property="og:site_name" />

[omarvelous]

Concur. Experiencing the same issue.

[kpumuk]

Fixed in master