To get around this in the Modern Designer, we've been using Rich Text fields, which do actually autogrow by default. We've used these lines in CSS to make the Rich Text fields look like a Multi-line field:

.rtfHideToolbar  .note-toolbar {
  display:none; 
} 

.rtfHideToolbar  .note-editable {
  min-height: 100px !important; 
} 

With this code, you would enter "rtHideToolbar" as the CSS Class in the field (on the Advanced tab of the field properties).

Hope that helps!