I think that they are still using the much criticised Covid space allocation plan.
I'm not sure that's the case - there's a consistency in travel being offered via Trowbridge but not via Chippenham. The tickets are described as "via Bristol" ... and Melksham to Bristol journeys are permitted either way.
I doubt whether there would be an issue if I got a ticket from the
TVM▸ and travelled (shock, horror!) via Chippenham - looks like a quirk in the booking engine. Mind, I was once required to show my ticket to one of the platform staff at Bristol Temple Meads when I asked whether I should change at Chippenham or Trowbridge when headed for Melksham - I asked why she needed to see the ticket and she told me to see which routes it was valid by - so there must be some ticket or other which is restricted, but I don't know what it is ...