I am running Photo Mechanic Version 6.0, build 6097 (85d1687) on Mac OS 12.1 (Monterey), MacBook Pro (16-inch, 2019).
I have a folder with .heic and .jpg files. The thumbnails are not getting combined. I believe I have correctly set the corresponding preference. See the attached screen clip to see an example of two thumbnails and the preference pane.