I went through a nether portal, and most block lighting reset to 0.  Sky light is fine.  I had to download the world to optimize it and clear the cache data (because I can't just do that on a realm world) and that fixed the light, but the next time I went through a portal the same thing happened.

I went into the overworld, haven't tested in End or Nether, only went through Nether portals.

edit: at first, only one of the portals did this, but gradually all portals, except for spawn chunk portals and the portal in the chunks I first logged in near when the realm first updated to 1.14