Not sure if this is a bug, but you can’t paste markdown into WP 6.3. I mean you can paste it in the editor, but the page will be all screwy.
Also, all old posts where you previously pasted markdown are similarly messed up, many block errors, uncentered images, can’t edit, etc.
I replicated this by creating a new post and just adding one block at a time, everything works fine.
I then created a new post and pasted in markdown like I have been doing for years all the way up through WP 6.2.2, and the post was completely messed up and uneditable again.
I sure hope they fix tihs! I write all of my posts in Obsidian using markdown and then paste them into the block editor. If I can no longer do this, no way I’ll ever upgrade to 6.3 again … had to roll back to 6.2.2 to fix everything.