The TextFormatter used for string and text fields behaves differently than the core formatters for the same fields. For example for Text (formatted) fields, all the allowed HTML is escaped.
Example <em>emphasized</em> title.<em> tags escaped, due to Twig's autoescaping.Use the same render arrays than core to display these fields, adding the heading tags as #prefix and #suffix.
It also has a slight performance benefit, because the Twig template is not used. I also included another minor performance tweak, which is setting the $size variable outside the foreach loop.
As a footnote, in my opinion this also has an extra benefit, and it's that you will be able to override the heading Twig template affecting only Heading fields, but not the core ones, which I think is preferrable.
Needs work
1.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.
Issue #3279535 by gonssal: Properly support string and text fields...
@gonssal : thank you for the suggestion and patch.
I altered the code: I added a html_tag render array around the string & text render arrays.
The change is merged into the 2.x branch.