2nd part to https://devrant.com/rants/1986137/...
The story goes on...

After I found more bugs that seem to be related to the communication break, and took a closer look, I sent detailed logs of my research and today we had a conference call.

"We have 2,5 million user, our system is widely-used and there is no plan to change it" they said.
And "We cannot reproduce the issue, but even if there is one, you will have to work around the problem, because we cannot make changes on our side" was one answer
As well as "If we would make changes, we will have to re-certify everything"

So I said we told 'em about the issue to let them improve their system. And I can work around it, I already figured out a solution for my side, but if there is a bug, they'd better fix it for future releases.
And with my additional research I have a bad vibe of some kind of memory leak involved on their "certified" implementation, and that could trigger various other problems.

But it is as always, if I try to be nice, I just get kicked in the ass. I should really be more of an asshole.