I suspect the Valley Lines closures for electrification may also have had an impact on numbers. For example, Treherbert to Pontypridd is closed from 24/04/23 until Feb 2024 for re-signalling, re-railing, extra passing loop, and OLE▸ . While there's a rail replacement bus, many will have reverted to car or used the "ordinary" bus.
And even for those using the replacement bus, I'd hazard a guess a significant number of those journeys are being done without tickets given the bus drivers have nothing to do with ticketing and there's no staff checking tickets.