Q: When I perform a keyword search, recently uploaded (and tagged) photos aren't included in the results.

Photos added to the library after the conversion don't appear in the search results.

Please clarify - are you adding the photos to the Photos Library or the iPhoto Library? And which application are you using to searchß

Keywords you add in Photos to photos in your Photos Library can be used for searches in the search field and in the smart albums.

Sometimes Photos has to be closed and relaunched in order to refresh the database files and allow searches, etc. to complete successfully.  Give that a try:

Try repairing the library according to the instructions in this Apple document: Repair Your Library.

Warning: If you use iCloud Photo Library with Photos, this action will cause Photos to upload all your pictures again (from Fix missing or incorrect thumbnails in Photos - Apple Support).