As @bmc38119 pointed out, the iframe didn’t fully make it to 6.3 → Miscellaneous Editor changes in WordPress 6.3 – Make WordPress Core.
Basically, the Post editor isn’t iframed if blocks with an API < v3 are in use.
All Cwicly blocks and Core blocks are now using v3. This is all fine until you realise that the iframe is cancelled as soon as custom fields / Meta Boxes are in use (i.e. ACF).
Custom breakpoints are planned to come out after the Tailwind integration.
Cheers,