Found it! The attached patent application is directly related to the original patent at the beginning of this post but has been updated to describe operations in a series/parallel HEV. It was just published on the 09/20/2018. What is really funny is that Honda seems to know that customers would freak out when the ICE comes on and specifically talks about how all the steps they take to prevent the use of engine braking until it is the last resort. (Bold emphasis is mine)
Section [0049]
In the second braking control illustrated in FIG. 4B, the second motor generator MG2 operates as a generator by the braking force of the vehicle, and the first motor generator MG1 is subjected to power-running by the regenerative power generated by the second motor generator MG2 to reverse the engine ENG. At this time, the engine ENG functions as the load of the first motor generator MG1, and the regenerative power is consumed for the power traveling operation of the first motor generator MG1.
Section [0003]
In a hybrid vehicle described in Japanese Patent No. 5965459, when engine braking is started after charging a traveling battery, after a braking situation immediately continued thereto, for example, after a braking situation when a hybrid vehicle goes down a slope or sloping land immediately after the start, there is a case where a driver misunderstands that no pure electric operation mode occurs.
Section [0005]
The hybrid vehicle disclosed in Japanese Patent No. 5965459 does not inhibit the engine brake when charging of the traveling battery is not permitted, that is, it is considered that the engine braking is performed. Therefore, when going down a slope or a sloping land after charged until the charging level of the traveling battery becomes a value greater than the charging threshold value, noise due to engine braking is generated. This noise causes a sense of incompatibility to the driver who tends to think that the quietness of the hybrid car is high due to the electric driving. In particular, since the driver is apt to feel a sense of incompatibility with the engine sound generated immediately after the charging of the traveling battery has been completed, the traveling comfort for the driver is deteriorated.
Section [0006]
An object of the present invention is to provide a control device for a hybrid vehicle which can improve driving comfort for a driver even when charging of a battery cannot be permitted
Section [0012]
the engine is utilized as a load of the first motor generator, is executed when a variable representing a charging state of the battery by a level of a value is equal to or greater than a predetermined value and use of a mechanical brake of the hybrid vehicle is restricted.
Section [0018]
...Therefore, even in a state in which charging of the battery cannot be permitted, if the use of the mechanical brake is not restricted, it is possible to improve the traveling comfort for the driver, without causing the driver of the vehicle to feel uncomfortable.
Section [0058]
Further, it is to be noted that the present invention is not limited to the above-described embodiment, but may be appropriately modified, improved, and the like. For example, the vehicle is not limited to the HEV capable of switching between the series type and the parallel type, but may be a series type HEV or a parallel type HEV.
In summation, the system is working as designed - and Honda knew it would cause discomfort. Instead of explaining clearly (in the owners manual perhaps) to the customer that engine braking has to happen from time to time, they take steps to reduce the number of occurrences. What they did not understand is that this tends to create more confusion and anxiety in the customer because it causes the ICE behavior to be sporadic and not wholly predictable. So the customer is confused because logically they think if I do this (full charge) then I do that (go down a slope) then it should always do this (ICE turn on). Since that is not always the case, the customer thinks something is wrong with the car.