You need to keep in mind that security disclosures and everything around (legal, why disclose? etc) it is a black box for most folks.
Also OP said after reporting it, "months later" OP was fired. This means anyone in those months anyone could've found what the OP found and abused it. And the effected bank might've not been involved until later? And all those crimes are now on your shoulders. Your then bank employee would get delayed through the politics in your company (even with good intentions). It will happen even for their own security disclosure handling. Let alone for competitors cos so much legal stuff are involved now.
From the effected bank's POV, you are just a bank employee who did some fraud. Mainly cos you didn't inform the effected bank and it's been months. Insider hacks from the same industry is not unheard of.
Like I said, this would've been a different scenario if you informed them ASAP. And I am sure when issues hit the roof your employer would wash their hands off. It is the easiest thing to do. Unless you are very well connected in the office. Otherwise even your employee are not sure about you. They will be asking questions like, why is this person doing this anyways? He is not in security so why was the OP poking around it? etc etc.
Hope this gets sorted out. Interesting case indeed.