For some reason the font Source Sans Pro regular weight option is not showing up in Premiere Pro CC 2018 - running Mac OS 10.13.5. The font is installed. All weight options show up correctly in other programs including Photoshop CC and inDesign. Of course, the client wants Source Sans Pro regular for lower thirds. I realize I could create them in Photoshop and import - which is what I'll do for now, but it's an annoying workaround.
I'm having this exact same problem. Regular and Bold fonts are fine in client-provided Illustrator file but I can only access Source Sans Pro Italic in After Effects, even though they're installed on the system. On Mac, used Find to locate three AdobeFnt_.lst and deleted them and restarted system (the folders mentioned in this post don't exist on my system running CC2020). Didn't fix the problem.
Having the same problem, too. And the Regular font in the Source Sans Pro family disappeared about the same day that yours did in both After Effects and Premiere Pro. I've tried every fix Adobe has given me, and chatted via messager and phone probably six times. Tried older versions, newer versions, etc., and nothing is fixing it. This is getting to be just a little bit (read: a lot) ridiculous.