Underscores not allowed in style key field.

Yeah, that seems reasonable. We're basically relying on Drupal's Html::cleanCssIdentifier to validate class names, but surprise (to me at least), it filters underscores by default.

Should be easy enough to refactor. I'll try to get something up soonish. Thanks for opening this!

I'm working on 📌 Add support for new field selection UI Active , and it makes sense to go ahead and fix this in those changes. Closing and adding credit there. Thanks!