darktable does not automatically watch folders for new files. Imported folders are treated as film rolls stored in darktable’s database, so if you add images to that folder outside the program, the usual way to make them appear is to import that folder again.

So, based on the community answer, there is no simple refresh button for this. Re-importing the folder is the expected workflow.

This database-based approach also has some benefits: darktable collections are not tied strictly to the file system, and exported files such as JPEGs can go back into the original folder without necessarily cluttering your working view.