It looks like this is fixed in the latest ACF release, as per patch notes:
https://www.advancedcustomfields.com/blog/acf-5-8-5-release/
*Release Date - 8 October 2019*
* New - Added new choice "Add" to the User Form location rule.
* New - Optimized acf_form() logic when used in combination with acf_register_form().
* Fix - Fixed bug causing incorrect field order after sync.
* Fix - Fixed bug reverting the first field type to Text in Firefox version 69.0.1.
* Fix - Fixed bug causing tinymce issues when changing between block modes.
* Fix - Fixed bug preventing block registration when category does not exist.
* Fix - Fixed bug preventing block registration when no icon is declared.
* Dev - Added RegExp compatibility for innerBlocks.
I am currently downloading to verify locally.
