Similar issue here, although I managed to get a cloud save in the ps4 version three days ago. Back then it didn't show up in the ps5 version though.
Today however the cloud save suddenly showed up in the PS5 version. It's three days old though and currently I'm trying to get an updated cloud save in the ps4 version, no luck yet.
I started a new game on PS5 once to get the "Load Game" button in the menu and an auto-save from the intro of the game showed up this afternoon, but now it's gone again.
I've noticed that the two occasions a cloud save showed up was at around 1pm CET.

Cross progression is really a big mess it seems.

Similar issue here, although I managed to get a cloud save in the ps4 version three days ago. Back then it didn't show up in the ps5 version though.
Today however the cloud save suddenly showed up in the PS5 version. It's three days old though and currently I'm trying to get an updated cloud save in the ps4 version, no luck yet.
I started a new game on PS5 once to get the "Load Game" button in the menu and an auto-save from the intro of the game showed up this afternoon, but now it's gone again.
I've noticed that the two occasions a cloud save showed up was at around 1pm CET.

Cross progression is really a big mess it seems.

Yeah, cross save isn't working for me either.
I received an error message a few weeks ago on PS4 telling me that services are down and since then I only occasionally see a cloud symbol next to my save files.
I received my PS5 a few days ago and no save file showed up in the PS5 version yet. 2 days ago while playing the PS4 version the cloud symbols suddenly showed up again so I quickly switched to the PS5 version but no luck. Cloud saves disappeared again on PS4 version.

Ubisoft Connect tab is working btw and I was able to unlock some bonuses.

I'm guessing they're trying to fix the corruption bug so they temporarily disabled the cloud save service or something like that.