It has been like this for a very long time.
OP provides functionality for users to send their images to external editors but, when you modify an image in some external editor and save it back with the same name as the OP used, the thumbnail may or may not update to show the changes and there is no way to force a refresh without either changing folders or exiting and restarting Optics Pro, and even then sometimes that is not enough.
I have reported this as a bug and was told to not return images from an external editor back to OP for further processing. Sort of like that old joke - "Patient: Doctor, it hurts when I do this. Doctor: Then don't do that". The Windows version has a refresh option for the folder but the Mac version does not.
While OP has a problem displaying changed images it does generally recognize new images in an existing folder and I have had success with saving any externally edited images with a different name. I wonder if changing a folder name would also fix your problem.