Can you delete and re-add photos?

I'm migrating to Photos.app and after importing a few thousand photos realized I needed to tag (using a Keyword) some of the photos in the old app, Mylio. So I deleted those photos in Photos and tried to import the same folder, but now some of the photos are tagged. Photos originally said that they had already been imported. Same name, but not several hundred had been retagged. After several reboots and half a day and retries, I was able to select the photos for import. I don't yet know if it's working. I search for the new tag and only two photos show up. But I also see indexing.

Shouldn't deleting photos just do that or does Photos remember too much? This is probably complicated with Photos being on iCloud.

FWIW the keywords were to mark albums. I didn't see any way to migrate Albums from Mylio, so created new keywords to mark all the photos from the Albums. Plan is in Photos to select all with that keyword and create an album.

Mylio photos are stored in a master folder which can be accessed with the Finder and any changes to the photos are saved and picked up by Mylio. One of the reasons I used Mylio, but it has other issues, hence the attempt to migrate to Photos. I was originally an Aperture user and when it was announced that Aperture would no longer be supported Photos was too lacking in features I went with Mylio, but I think Photos will be adequate now.