ACF have published an article that explains the cause of the issue and migration options, including a v2 compatibility plugin.
https://www.advancedcustomfields.com/blog/acf-6-8-9-released/
For my purposes setting blocks to v3 and disabling inline render previewing is the cleanest migration path for blocks that previously had inline editing and no preview:
“acf”: {
“blockVersion”: 3,
“renderPreview”: false,
“renderTemplate”: “blah.php”
},
