Unfortunately I think you've already found the best Workaround.
Here's what has worked for me. Took me a long time to find this. Stickies 'help' is no help at all.
First, select the text you want to change, then, control click to bring up the contextual/pop-up menu
and scroll to font > styles... Click "apply" if you see a style with single spacing (Spacing 0.0x). You can search through the various syles: Document Styles or Favorite Styles. I usually select Favorite Styles and then "default" then "apply".
There are sometimes (depending on the note) multiple styles in a note that was pasted from another application. This particular menu has other useful things in it as well, such as formatting a list.