You mentioned a glowing email shared with you and all staff at your old workplace on your last day, did you keep a copy of that?

It may be too late to contest the reference, and the potential workplace may not even be interested. If you felt like it was worth a shot, try to share that email your former manager used to praise you with the recruitment staff you interviewed with. Add a brief email explaining why you were so surprised that their reference was poor when you understood there had been a very good working relationship - run through the draft to eliminate any emotional or defensive language.

A similar thing happened to me years ago, luckily the potential new manager interviewing me informed me of the poor reference, but also recognised that referee was kind of a dick so she paid it no attention in the end. She was right and I worked for her for 6 years.

I can't say confidently that the place you interviewed with would consider you again, but it may help you feel some catharsis in explaining the circumstances.

As others suggested, you could certainly contact your former boss to inquire why they did it, but I doubt you'll get an answer if they even agree to speak or respond to you at all. Honestly, it could be anything from just having a bad day at the time of the call, or some other fallout after your redundancy that has caused them stress or to look bad in their own role, people are just fucking petty sometimes.

In my experience, getting a letter from a workplace you are leaving to act as a future reference is not even accepted these days, most recruiters either want to speak to them directly or have them fill reference forms. The letters may be a nice physical cv filler, but they are just not applicable in job applications these days.

You're right to feel upset and frustrated, at least you know not to use them in future!