Note 5 running 5.1.1
Here is my issue...
A .nomedia file placed in a folder that automatically downloads album art will only hide the images from Gallery if they were in that folder before the .nomedia file was created or modified. Any newly downloaded album art is visible in Gallery. Clearing the cache, data, and force stopping both Gallery and Media Storage apps, as well as shutting down and restarting the device does not correct the problem. The images downloaded after the creation/modified date of the .nomedia file will be visible no matter what.
Perhaps this is how .nomedia files work and I just never really understood them, but I thought they would hide all media files in the given folder no matter when those files were added to the folder. I didn't think the date of the .nomedia file had any bearing on it.
If anyone else has experienced this and found a solution I'd like to know! And I do not consider installing alternative gallery apps that can hide images a solution to this.