Update to Search API 1.18 causes HTML to be displayed as plaintext
check_plain() encodes special characters that has special meaning in HTML (such as &lt; and &amp;) into plain text entities (i.e. &lt; and &amp; respectively) that will make these be rendered literally (not interpreted as HTML) when that string that is then displayed as part of a page with HTML-markup. 
filter_xss() filters an HTML string to prevent cross-site-scripting (XSS) vulnerabilities.

In other words, filter_xss() takes care of security, while not plain-texting HTML.

I propose you to patch the Search API function with 'filter_xss' as a callback instead of 'check_plain', create an issue on drupal.org and explan that case.