'Check Hybrid System' message and carbon build up

First time posting. The dreaded 'Check Hybrid System' came up with the engine becoming sluggish and almost stalling.

Have sent it to my local dealer. Their diagnostic came back showing carbon build up. Decarbonization required. Hybrid battery good. Actually the Prius went for a major service a few weeks back with clean bill of health for the batteries.

This is the second time this have happened. The first time around 3 years ago, decarbonization resolved it. Car has 135k km (83k miles) on it. When the first time it happened, the dealer advised to take it out for long drives. I have been taking it for 2 hours drive once or twice a month. Most of the time the car is used for taking kids to school around town. Looking at 10 km every day.

Anyone else have this problem? Don't see anyone else having this issue on the forum. I thought a small engine with hybrid wouldn't have carbon buildup issues?

Thanks!