During testing of jsc#SLE-2995 and playing editing form.yml to create different visualizations, $help and $placeholder could not be displayed (even without $default value).
It is easy to check using a small portion of this example:
https://github.com/yast/yast-configuration-management/blob/master/test/fixtures/formulas-ng/test-formula/form.yml

Expectation:
According to https://www.suse.com/documentation/suse-manager-3/3.2/susemanager-best-practices/html/book.suma.best.practices/best.practice.salt.formulas.and.forms.html#best.practice.salt.formulas.pillar

"$help and $placeholderThe $help and $placeholder attributes are used to give a user a better understanding of what the value should be.$help defines the message a user sees when hovering over a field$placeholder displays a gray placeholder text in the field$placeholder may only be used with text fields like text, password, email or date.
It does not make sense to add a placeholder if you also use $default as this will hide the placeholder."