The usual behavior of Lightroom is that when you automatically tag a Person based on facial recognition, the keyword associated with the person is added to the keyword list. As of the April 2018 Lightroom CC Classic Update (Windows 10), this had stopped happening. I have a whole folder with correctly face-tagged photos, but these photos do not automatically receive the proper keyword. If I run "Find Faces Again", it does seem to fix it. If I manually add a Face, or highlight an existing Person tag during face view then press "Enter", I will then see the proper keyword populate. It might have been a transient bug, but make sure to check your new folders that you face-tagged around April 2018 and ensure that the keywords are present.
We're really sorry for all the hassle. Could you please let us know the exact version of Lightroom you are using now? Also, are you facing this issue with all the Lightroom catalogs?