Sorry about the double posting however I did see a YouTube video called PTV sign on program break (WNET 1999) and I found out because of the glorious gospel promo and the nature Iceland fire and ice documentary promo they aired February 3rd and 7th 1999 and seeing some of that rare intro you were talking about they used the intro February 1999 so the latest the intro could’ve debuted actually could be February 1999 or maybe even January 25th 1999 because that program break was exactly one week after Zoboomafoo premiered which was January 25th. Here’s the link for proof credit to doofam2 of YouTube and gamehero who originally owned the tape youtu.be/17wTFgbgMQM
I've also considered the possibility that only some PBS stations received the Johnny Kemp intro, to help test how well or poorly viewers would respond to it. I imagine in this case, the other stations would've continued to use the Tina Fabrique intro, until PBS started rolling out the one with Chaka Khan.
This seems to be confirmed by this 2000 program break which shows the Tina Fabrique intro:
Of course, I also saw a commercial break from 2002 which used that intro, so it's possible it was still used occasionally but had stopped being used exclusively in 1999.
BTW, I'm hearing the Johnny Kemp one premiered in October 1998?? If it lasted up to Setptember 2001, that would mean it was used for almost 3 years. It's strange that it would still be so obscure after almost 3 years of airing. So I'm thinking it's most likely that some stations never used it.