This is so weird:
Working:
ACF 5.8.0-beta1 /WP 5.9.8 / Gutenberg 4.0.0
NOT working:
ACF 5.8.0-beta2 / WP 5.9.8 / Gutenberg 4.0.0
Working:
5.8.0-beta2 (var $version changed to 5.8.0-beta1) / WP 5.9.8 / Gutenberg 4.0.0
NOT working:
5.8.0-beta2 (original) / WP 5.9.8 / Gutenberg 4.1.1