Experience Builder does not support blocks which are not marked as validatable by config schema. Unfortunately, Simple Search Form falls into this category - it's completely hidden from XB.
To fix the bug, add this to the block.settings.simple_search_form_block config schema type:
constraints:
FullyValidatable: ~
Problem solved.
Active
1.0
Code
To clarify "marked as validatable by config schema", FullyValidatable is an opt-in flag to stricter validation, see
https://www.drupal.org/node/3404425 →
Automatically closed - issue fixed for 2 weeks with no activity.