Denoised (DNG) images not staying in catalogs

I've been having an issue where my desoised, DNG, images don't appear in my catalogs after returning to lightroom. For example: I denoise and create a number of new DNG images while editing, they all appear in the catalog where I created them, I shut lightroom down for the day, and when I return the DNG files are no longer in the catalog. I have to go to "all photographs" and pull them into the catalog manually. 

For troubleshooting's sake I have ensured there are no filters on and none of these catalogs are "smart." I also don't "stack" the images. After pulling them images back into the catalog the number of images changes, which is an additional idicator that the images left the catalog altogether. 

While this isn't a huge deal, it does negatively affect workflow and is a time consuming process for somthing that should be automatic. Any assistance would be greatly appreciated, thank you!