A library I built in earlier 1.14 snapshots and completed by Pre-Release 2 functioned perfectly by having glowstone under lecterns as a way to hide light sources.

After updating to Pre-Release 3, if the lecterns are interacted with at all, they start blocking the light source underneath them.

In my case, this totally wrecks how they previously functioned for me up until this point and I'm honestly not sure how to relight this area to make it safe again.