I assume you're referring to an online booking engine that's giving erroneous fares.
Just buy the Anytime Day Single (£4.20) from Montpelier to Pilning on the train from Montpelier on Saturday.
There are Routeing Guide easements, both local and fare check, that allow you to double-back from Newport to get to Pilning. These should be programmed into booking engines, but may not be implementing correctly.
You shouldn't have any issues using the Anytime Day Single. Staff should be well aware of the limitations of getting to Pilning from the Bristol area with trains only calling in the up direction.
700765 Local
To allow journeys from Severn Beach, St Andrews Road, Avonmouth, Shirehampton, Sea Mills, Clifton Down, Redland, Stapleton Road, Montpelier, Lawrence Hill, Filton Abbey Wood and Bristol Parkway to Pilning via Newport (South Wales), this local easement will apply on Saturdays.
700772 Routeing Point
To allow journeys on Saturdays from Severn Beach, St Andrews Road, Avonmouth, Shirehampton, Sea Mills, Clifton Downs, Redland and Montpelier to Pilning via Newport (South Wales) this routeing point easement will override fares checking.
Are you planning on returning by train from Pilning? If so then it's the £6.50 Anytime Day Return, and don't miss the later departure! Or is the plan to reposition to Severn Beach under your own steam to get back home?