Cruise control behavior

Every one of the few times CC disengaged for me there was a message on the MID indicating why it disengaged. eg. "No Longer Following a Vehicle" (or similar) is the one I've gotten the most when the car in front turns off the roadway.
I think the manual warns you are really not suppose to use CC in stop and go traffic except on Expressways.
I use it mostly to keep from exceeding the speed limits as my Car Insurance App dings my score for speeding.

 

From the manual:
Approach warning (vehicle-to-vehicle distance control mode)
When your vehicle is too close to a vehicle ahead, and sufficient automatic deceleration via the cruise control is not possible, the display will flash and the buzzer will sound to alert the driver. An example of this would be if another driver cuts in front of you while you are following a vehicle. Depress the brake pedal to ensure an appropriate vehicle-to vehicle distance.
■ Warnings may not occur when
In the following instances, warnings may not occur even when the vehicle-to vehicle distance is small.

● When the speed of the preceding vehicle matches or exceeds your vehicle speed
● When the preceding vehicle is traveling at an extremely slow speed
● Immediately after the cruise control speed was set
● When depressing the accelerator pedal​

 

Sorry, I meant to post this about cancellation of DRCC from the manual

Automatic cancelation of vehicle-to-vehicle distance control mode
Vehicle-to-vehicle distance control mode is automatically canceled in the following situations.

• Actual vehicle speed falls below approximately 25 mph (40 km/h) when there are no vehicles ahead.
• The preceding vehicle leaves the lane when your vehicle is following at a vehicle speed below approximately 25 mph (40 km/h). Otherwise, the sensor can not properly detect the vehicle.
• VSC is activated.
• TRAC is activated for a period of time.
• When the VSC or TRAC system is turned off by pressing the VSC OFF switch.
• The sensor cannot detect correctly because it is covered in some way.
• Pre-collision braking is activated.
• Intelligent Clearance Sonar is operated. (if equipped)
• The parking brake is operated.
• The vehicle is stopped by system control on a steep incline.
• The following are detected when the vehicle has been stopped by system control:
  • The driver is not wearing a seat belt.
  • The driver’s door is opened.
  • The vehicle has been stopped for about 3 minutes.
    In this situation, the shift position may automatically switch to P. (→P. 333)

If vehicle-to-vehicle distance control mode is automatically canceled for any other reason, there may be a malfunction in the system. Contact your Toyota dealer.

Could your issue fall under the first bullet?

 

I "Believe" that "Engaging B Mode" should be added to the list.
It appears to me that Radar Cruise Control (maybe Cruise Control too) is/are mutually
exclusive of "B Mode".
I had DRCC disengage a few times (I don't believe there was any warning/message) when I
engaged "B Mode" and when I was already in "B Mode" I could not engage DRCC.

 

Yes, that is true. Neither cruise control option works in B Mode.