Recall this being an issue in one of the early Pro 4.0 betas and it looks like it is back. The builder preview for the new GF RC-1 is not visible but still works on the front end.
Thanks for the heads up on this one.
Not sure if it is related but in the latest Pro 4.3.1 all GF forms (currently running GF 2.5 RC2) are not working in preview or live site if added after upgrading to 4.3.1.
I’m sorry, I’ve still not looked into this one yet. We did change quite a bit in the builder live preview for this release so I have an idea of where it could have broken.