As an engineer for an ISO compliant company myself, I’m very aware of change/version control. They have noticed that trucks before Jan 2020 have the issue, so clearly they know something is different in hardware or software. If you read the instructions, it says right in them that the brake lights will not work with the manual button. Clearly when those were written, the test vehicle they used was a later build that didn’t have this issue. It’s also very possible that engineers on the brake controller only saw updated diagrams/info and designed to that. If they had been unaware of the previous rev and its potential flaws, then they might not have been able to design around it. Since they were both Tekonsha made, that shouldn’t have happened, but the folks working on the Mopar controller may not be the same ones from the aftermarket line and therefore not in the loop on the 3073-P with diode.Again I wish I had full wiring diagrams for the trailer circuits as there are relays for the trailer lights - trailer park, left and right turn, backup, etc. - I see those can be set to active or inactive in the system.
Also - engineering would have known of the change if a diode was added because of change control, version control. They could look at the revision history and see it in a heartbeat.
Companies that make electronic things must keep strict histories on them, if they so much as change the company that makes the PCBs for them, they have to revision it.
When you create an electronic circuit, if you move a trace or add or change any component, suddenly that software ups your revision level and you have a rev'd product. It's automatic.
So if they added anything, it was done in the software, a component was added, a trace changed, they were forced into a revision level change. No way around it.
Especially if they sell to the EU.
ISO compliant companies have teams that do nothing but watch over revisions and the paperwork involved, and they are audited every so often.
If there was a diode added - someone had to request it, it had to be engineered, drawn into the PCB, which meant a revision change, then it had to be tested with that change, the company that makes their boards and electronics had to receive the change, and it had to be documented to the very first vehicle that got it.
Unless, as you said, we had access to the full wiring diagrams and such we don’t really have a way to know. A quick test for a diode would be easy to do and either prove or disprove this theory. Until one of those happens, and without appropriate diagrams, we only have educated guesses.