I did some research and found that in iPhoto Titles, comments, locations, and I believe faces are stored in the iPhoto library database and not in the image file itself. When importing iPhoto libraries into Photos apparently only the images are copied and not the library database and its attendent information. Information on Photos is sketchy so far, but I assume when that data is created in Photos it is stored in the Photos library database as well.
I have not yet been able to determine if the Photos library data is transferred to the iCloud or not. If it is, it does not appear that it works way down to the iOS versions of Photos.
I am still researching this.
It may get weirder than what you just described.
I looked at some iPhoto—>Photo imports in Photos and keywords are imported, along with the original title from iPhoto, but this title does not show up in the Title field in Photos; that field shows "Add a title". However, if you choose to manually export said photo, like to your Desktop, it appears in its destination with "original title.jpg" showing on the file.
Also, with Export… in Photos you can select to include "Title, Keywords, Description", which is how the original title (at least for me) showed up, even though the Photos Title field was blank. Go figure!
