I believe journey planners permit this on some journeys purely due to a potentially erroneous easement by GWR.
Easement 700885 states:
From 15 December 2019 customers travelling from Lawrence Hill, Filton Abbeywood, Ashley Down, Stapleton Road and via Stapleton Road to Bristol Parkway or via Bristol Parkway may doubleback at Bristol Temple Meads. This doubleback easement will apply in both directions.
My understanding is that this is intended to allow passengers travelling to or from these local stations to travel into Bristol and back out to Bristol Parkway for long distance services.
One of the electronic easement records, however, is written as follows:
If all of the below apply to the journey:
- passes through at least one of { Ashley Down [ASD], Filton Abbey Wood [FIT], Lawrence Hill [LWH], Stapleton Road [SRD] }
- passes through all of { Bristol Parkway [BPW], Stapleton Road [SRD] }
then the journey may double-back through { Ashley Down [ASD], Filton Abbey Wood [FIT], Lawrence Hill [LWH], Stapleton Road [SRD] }
Journey planners will interpret this as meaning that if
one of the trains you take between Bristol Temple Meads and Bristol Parkway
or Bristol Temple Meads and Filton Abbey Wood calls at Stapleton Road, it will permit the double-back into Bristol Temple Meads and back out to Parkway. However, if you take services that go non-stop through Stapleton Road, the double-back will not be permitted as the station is not a mandatory timing point for services that do not call at the station, thus the train does not "pass" the station in any way interpretable by online journey planners.
I would imagine this easement is not supposed to apply to this journey, but may be doing so erroneously and could be corrected in the future.