I wouldn't say it's lowest priority. Lowest priority would be cancelling service completely in favour of using trains or train crew on other routes.
They've done that too plenty of times.
The issue here appears to be reliability, whether that's congestion at Cardiff Central, Cardiff Queen street, or somewhere else along the route between Coryton, Penarth and Caerphilly that's causing these delays, which ultimately results in trains turning around short somewhere.
I think the issue is very much a choice of priority.
For one thing, it doesn't matter which train is late when there's a conflict at Heath Junction - it's almost invariably the train from Coryton that is held.
Often for 5 or 6 minutes because the junction has already been set for the main line.
Trains to Coryton are almost always turned short earlier than they needed to be.
Take today for example - a train arrived at Heath Low Level 9 minutes late and was turned around there, waiting 13 minutes.
Birchgrove is nominally 3 1/2 minutes away (in reality it will be less since Ty Glas is shut).
On Monday they did this for a train that was 5 minutes late meaning it sat at Heath for nearly 20 minutes.
I have seen them cancel two trains in a row completely and then turn the next one around short because it was 7 minutes late.
These do not look to me like the inevitable result of delays - they look like deliberate decisions to prioritise the rest of the network with little regard to the impact on services to Coryton.
If someone can explain why I'm wrong I would genuinely be very interested.