Custom post type fields group not rendering in Gutenberg editor

I am facing a persistent issue where a complex Meta Box field group ("Honvéd adatmezők", ID: honved-adatmezok) fails to render visually inside the Gutenberg block editor for its assigned custom post type (honved), despite being fully visible and active in the Meta Box builder backend.

Environment Details:
Custom Post Type: honved (registered with show_in_rest enabled)

Field Group ID: honved-adatmezok (structured with multiple tabs and fields)

Site Context: The site previously used the Classic Editor, but has been migrated to Gutenberg.

Tools Used: Meta Box AIO, WPCodeBox (for custom PHP snippets), Falcon (deactivated during testing).

Symptoms:
The field group exists, is active, and is properly targeted to the honved post type in the Meta Box builder.

In the Gutenberg editor, the screen options show that the meta box checkbox is checked, but the box itself and its fields are completely missing/not rendered visually.

Browser console logs show standard block warnings along with a specific notice regarding the iframe editor and Meta Box styles:
block-editor.min.js:83 mb-blocks-css was added to the iframe incorrectly. Please use block.json or enqueue_block_assets to add styles to the iframe.

Troubleshooting Steps Already Tried:
JSON & Builder Verification: Exported and verified the JSON structure; field definitions and tab setups are clean and syntactically correct.

Post Type & REST API Check: Verified and re-saved the honved post type and field group associations in the Meta Box UI to ensure database relations are intact. Confirmed show_in_rest is active.

Global & Targeted Filters Tested:

Forced block editor support using add_filter( 'use_block_editor_for_post_type', '__return_true', 999 );

Tested rwmb_should_show and rwmb_rest_api_get_post_meta_boxes filters.

Attempted disabling Meta Box block rendering via rwmb_use_block_editor.

Programmatic PHP Registration: Tried registering the field group directly via a PHP snippet in WPCodeBox using the rwmb_meta_boxes filter to bypass any potential corrupted database relation records (mb-field-group), but the box still failed to mount.

Conflict Testing: Completely deactivated performance/optimization plugins (like Falcon) that might interfere with script loading or Gutenberg asset enqueuing inside the editor iframe.

Could you please advise what might be blocking the Meta Box fields from mounting inside the Gutenberg/iframe editor environment given this migration background?

Thank you for your help!