Create and save a tag during backend article creation.
Tag class is empty.
Open/edit/save/close tag in backend.
Tag class is changed to "label label-info".
I have confirmed this also happened in prior recent versions of Joomla.
Joomla 3.9.6
This is an issue with your template. Although class is indeed not saved for tags created on the fly, the default tags layout uses label label-info class as fallback:
This is an issue with your template. Although class is indeed not saved for tags created on the fly, the default tags layout uses
label label-infoclass as fallback:joomla-cms/layouts/joomla/content/tags.php
Line 24 in 71266bd
Your template has a tags layout override (templates/shaper_helix3/html/layouts/joomla/content/tags.php) which does not use a fallback: