Okay, I finally got a callback from Adobe. It turns out that Premiere Pro is not designed to import SCC files UNLESS those SCC files were created in Premiere Pro.
I wish they had told me that up front. I do not think they make that clear in their documentation either — or at least I never noticed that clarification. Adobe seemed to be defensive about this. To confuse matters, they kept blaming it on my software, without telling me that PP won’t import SCC files from ANY outside caption creation system.
So in some sense, Adobe doesn’t use real SCC files at all, since legitimate SCC files cannot even be imported into Premiere Pro.
They use a format that is tweaked to be proprietary, I guess.
[sigh]