In Layout Builder, the preview for FieldBlock works on the "Choose block" dialog, but doesn't work after you've selected the block and are on the "Configure block" form. The same for if you edit an existing block.
FieldBlock
Active
3.0
Magic
Here's a patch to fix!
Tested this on Panopoly and a child distribution, it's working as expected.
Issue #3519757: Preview of FieldBlock doesn't work on the "Configure...