Not according to the Mixing Deck with the dates below:
Looks like the issue is with Southeastern, as doing a search for Taunton to Paddington is giving me Advances starting at £29.50 in both directions (from £61.50 for the outward journey and £50.50 for the return in First Class.)
I'll see if I can contact SE about the issue first, it definitely wasn't always an issue as I've used Taunton-Crayford advances plenty before, and they're a lot cheaper than an Advance to/from Paddington and Oyster the remainder (to the point that current Paddington Advances are only slightly cheaper in total than an open return for the full journey to Crayford).
I've managed to have a web chat with them, where they were unable to find advances for the full journey, and when I asked if there had been a policy change, they mentioned covid saw some additional advances brought in but have slowly been being removed. They offered to raise it with the Fares Team for more info but unfortunately the login somehow expired before I could saw yes, losing the chat line. I did also have a look at some random other SE only advance routes and it is possible to get Advance tickets on Mainline routes, and even Taunton to a mainline station seems to work as a through advance, some I'm wondering ifit's no longer possible to have an Advance ticket starting/finishing in the London travel zones for SE at least?