ACF 6.0 release announcement with ACF Block Type registration changes as they adopt block.json registration. ACF | ACF PRO 6.0 Beta 1 (advancedcustomfields.com)
The previous ACF Block Type registration approach used ACF | acf_register_block_type() (advancedcustomfields.com).
To support Inner Blocks the Block Type registration needs to have JSX turned on under the “supports” configuration in the “acf” section of the block.json file.
One of the ACF Pro scripts that handles Inner Blocks is wp-content\plugins\advanced-custom-fields-pro\assets\build\js\pro\acf-pro-blocks-legacy.js.