Back in NFM95 (January to May 2007), the Anytime Day Single (SDS) fares were identical and the Off Peak Day Return (CDR) fare to Bexhill was 40p more expensive than to Hastings:

Adjusted for inflation, the CDR fare to Bexhill would be about £18.30 in current money, which it is, while the Hastings fare is around £6 too high. Indeed, the Hastings CDR has been increased so much, it now costs the same as the Anytime Day Return (SDR), rendering it pointless.
In reality the problems here are
1) the CDR fares between Tonbridge (and anywhere associated with Tonbridge) and Hastings have become far too high; and
2) some fares have erroneously been restricted to travel via Tonbridge when it's not appropriate to do so.
The route restriction is very silly, and it could be an unintended consequence of the poor decision by the clueless people at the ORR to reduce the amount of "jargon" by adding daft route restrictions to tickets; see:
https://www.railforums.co.uk/thread...y-valid-only-via-newport.300635/#post-7738600
Clearly to any sensible person the Bexhill fares are correct, proper and how they should be. Sadly, Southeastern may see it very differently, and may think that the daft Hastings fares are correct. So if it was me, I wouldn't bring this to their attention, as you cannot trust train companies to resolve fares issues in a way that benefits customers. However, now it's been posted here, it will probably be seen by Southeastern.
The only saving grace is that Southeastern can't increase any fares at the current time, due to the current fares freeze.
You can buy the less restrictive ticket from the forum site, which will either give itineraries via Ashford or via Tonbridge, whichever is your preference. There is an "open return" option if you don't want the return portion to be associated with any given itinerary.