You are attempting to add a tooltip to a field label in Gravity Forms using a WordPress filter. The issue you are facing is likely caused by the fact that the field label is encoded as plain text instead of HTML.
Fortunately, there is a solution to this problem. Utilizing the gform_field_content filter instead of the gform_pre_render filter can help you modify the HTML output of a field. This contrasts the gform_pre_render filter, which only allows you to modify the field properties.