It says the seed is 0, not the real one

I really don't have a good answer to this. Of course you're not supposed to learn the seed of a random seed run until it's over, and downgrading can set the seed to 0 and erases whatever it was. This also is kind of a way to just break the random seed category because it becomes a set seed of 0. So maybe downgrades that destroy the seed shouldn't be allowed for random seed runs on that basis.

just make a backup before downgrading?

The problem is the seed is 0 after the downgrade. It's not just that it says it's 0. In this situation, the run has been played on 2 different seeds, one of which was known in advance, which makes it ineligible for random seed at all.