Gas engine doesn't come on to "dump charge" or "protect the battery". It comes on to stop the vehicle vehicle without using mechanical brakes. As at high level of charge it's impossible for battery to do that without using electricity (plugging) which is horribly inefficient. To regulate the rate of braking, using electromagnetic braking, the vehicle must be able to regulate the current flowing through the generators while braking.
For electromagnetic braking to work the battery must function as a load and be able to pull enough current to generate a magnetic field to stop the vehicle. Since the batteries have an exponential increase in resistance as they reach full state of charge (85+ percent) there is a point where the batteries physically can't sink enough current to stop the vehicle. It's not about protecting the battery, it just the battery resistance it to high at that point. If battery is full, it just not physically possible for it to pull enough current to stop vehicle. Different manufacturer's dealt with the physics of this in different ways.
In the case of the clarity the choices for the drive train is to use mechanical brakes, use plugging (consume electricity) or use the ICE. It's choices are more limited due to fact that it doesn't have variable gearing. It's braking behavior is a design choice and assume they looked at efficiency and wear on components etc.
In the Tesla they have vehicle only charge to 80% by default. This allows enough room to allow for electromagnetic braking through regen. When battery reaches full the vehicles only choice is to use the mechanical brakes. So Tesla tries really hard to NOT let battery hit 100%.
On Chevy Bolt the 100% charge isn't really 100% charge. It's closer to around a ~92% charge on the pack even though dash says 100%. If you happen to charge a Bolt to 100%(as indicated on dash), when you brake you get a white indicator on the dash instead of a green one. Which indicates the electromagnetic/regen braking is limited and the vehicle will depend more on the physical brake pads when the indicator is white. You also will find that when at 100% the vehicle will only use regen at lower speeds as the battery can't function as a large enough load at higher speeds (current would need to be much higher).
On the Chevy Volt at a 100% state of charge, (depending on which gen you have) there is a point where the regen paddle does nothing when coming down a hill. You can tap it, and you don't slow down. You just get a warning. You have to use the brake pedal to slow down, which uses a little bit of regen but mostly uses mechanical braking.
BEV can pull the trick of not really charge to 100% to allow electromagnetic braking because the batteries are big enough that the missing range is built into the design and doesn't negatively effect the owner. In the PHEV they are trying to get every last mile out of the pack, as it's much smaller, so they tend to result to tricks like plugging or using the ICE to sink enough current to allow vehicle to stop. As the "feel" of the mechanical braking vs electromagnetic braking is usually significantly different and they don't want to have a negative effect on the driving experience.
There are some other limits you might see when near a 100% charge on the clarity. At 100% charge, at high speed, even with ICE on, the number of deceleration indicators on the dash you see from the deceleration paddles may often be less than when battery isn't at a full state of charge. You can find you have one less than if the battery wasn't at a 100% state of charge.
