Imported photos not showing in Photos > Imports

Recently I imported several photos from Mail to Photos on my iMac (10.14.6) by right-clicking and selecting Export to Photos. When I opened Photos, there was no notification of the import and nothing in the Import tab. When I tried again using both "Export to Photos" and a click-and-drag to the Photos icon, I received a notice that these photos were already imported. I looked around and finally found the photos in My Photo Stream. I then restarted my iMac and the photos finally showed up in the Imports tab. Is this just a one-time glitch, or should I be doing something different regarding importing? Is there any routine maintenance/indexing that needs to be done to the Photos Library?