I contacted Ninja Forms support and this was the reply:
“I apologize for the inconvenience. Ninja Forms version 3.6.10 includes a security patch to prevent unwanted scripts from running on your forms, and unfortunately a consequence of this update is that HTML is no longer parsed in field labels. We’re working on a larger solution that would let you still place HTML in field labels while protecting you from malicious actors, and we hope to have this ready to implement soon. For now, you can continue to use version 3.6.9 while we complete that bigger solution.
I can’t promise what will be in the next version or when or under what circumstances HTML will be functional in labels again, but it’s an area of ongoing discussion with our development team, and we hope to have a solution ready to implement as soon as possible.
The 3.6.9 version of Ninja Forms core plugin can be downloaded at the bottom of this page: https://www.ads-software.com/plugins/ninja-forms/advanced/”
I used the WP rollback plugin to re-install v3.6.9 and disabled auto updates for Ninja Forms.