That's highly unusual. I believe you, but it's definitely a weird breakdown in the process.
It'd have been easier, given you were working on it before you joined the company, to claim it on the carve-out document that described the blanket copyright policy. That's what I did for my projects and it was never contested.
(And to be clear, I find the experience you had completely unacceptable. If I'd had the experience you had when I was working there, it would have made me significantly reevaluate whether I wanted to keep working there).
It'd have been easier, given you were working on it before you joined the company, to claim it on the carve-out document that described the blanket copyright policy. That's what I did for my projects and it was never contested.
(And to be clear, I find the experience you had completely unacceptable. If I'd had the experience you had when I was working there, it would have made me significantly reevaluate whether I wanted to keep working there).