I observed this behavior in PhotoLab 5 and still on PhotoLab 6.8.0:
In PhotoLibrary-view open a folder with processed images, then set filter to show only processed images and open one for editing (Customize-view). Now if any of the settings is changed the selected photo abruptly disappears from the Customize-view and is replaced by another one from the filtered PhotoLibrary-list.
When a photo is opened for editing, it should not change unless the user requests a switch to another photo. An abrupt switch when the photo status no longer matches the underlying filter does not seem like a rational design choice.
“Processed” means something like “exported” and as soon as you change one slider, the newly customized image disappears…because it has not been processed/exported yet: Changing a setting simply adds an entry to the “recipe”, along which DPL will process/export the image, once you tell it to do so.
I agree with @Rokka that this is undesirable behaviour and I have noted it in these forums before. A photo opened in Customizer should remain open in Customizer until closed by the user. The Customizer should not be subject to any filters — those are for the Photo Library.
It is particularly annoying considering PL will also refuse to read metadata on an image it creates itself*.
So in one case, it is “instant dismissal” when the user might least expect it, and in the other, it forces the user to take actions to include an image which should be there automatically.
*When you choose Export to Application on a photo, the created file has the same metadata as the original, such as the colour label. However, on returning from the external application, PhotoLab has not read the metadata from the file. So filtering to “green” images, then editing one in an external application, then returning to PhotoLab, the new image is not shown. I have to reset the filter, click the squiggle to force PhotoLab to read the metadata, then re-apply the green filter.