While WordPress 5.9 will be great when they get the bugs out, at the moment it has a whole lot of bugs. Listed here as I notice them. The Fixing WordPress Forum on WordPress.org is buzzing.
- Drag-and-drop rearrangement does not work in the new gallery block.
- Turning off full screen in edit mode, which I like to do, results in some pop-ups being shoved too far left (like the add link pop-up)
- All plugins will probably be scrambling to update in the next few days. Every one of them currently has a warning (not tested with your current version of WordPress) on it.
- Something went wrong with my sidebars and headers. But this did not happen another sites that use Blocksy and WordPress 5.9, so I am not sure why it happened. I reset all the customization options, which was a pain.
I think probably I should not allow such a major upgrade to occur automatically. It will clearly take a few days for things to settle down.