Same issue. I have a pbr resource pack in my global resources. Make new world in preview, activate experimental features, activate pbr resource pack (tried both as global resource and specifically just for world), and just like you the "deferred technical preview" option is greyed out.
Same issue. I have a pbr resource pack in my global resources. Make new world in preview, activate experimental features, activate pbr resource pack (tried both as global resource and specifically just for world), and just like you the "deferred technical preview" option is greyed out.