This problem exists since F1 2016.
Why can an engine customer team like Haas, Sauber (now Alfa Romeo), McLaren, Williams etc. develop the engines by themselves? Everyone wants more realism in the game, but this is not really realistic.
It would be better if you as a customer team could "help" the factoryteam develop the engine by releasing engine-telemetrydata or testing a package of engines a week or two earlier, with (perhaps) less reliability. So you have to wait until the factory team gets a engine update (like in RL). That would also mean more competition, right now you can develop in a season from the bottom up to the top.
It's different with transmissions, because some customer teams still develop their own transmissions. so the current development system is very unrealistic ...