Turns out this was two completely separate issues. One for core/plugin stylesheets in development, and one for child theme stylesheets. Both should now be fixed.
- No live reload when editing core/plugin scss, if you were using a non-default theme in development
https://github.com/discourse/discourse/commit/16fbbd096a6afbd7eefc1b8dd8048a8ad9474779
- Parent themes were not refreshed when editing child themes
https://github.com/discourse/discourse/commit/260398d706627f89765548f9cbd4eed2c1701968