I don't regularly check these forums or bug complaints, so I don't know if this has been brought up before or is a known issue. I also don't know whether this is something inherent to the fact that it is a snapshot, and wouldn't be a problem in the full release of 1.14, but the fact that smooth lighting is noticeably worse in the 1.14 snapshots concerns me, considering that 1.14 is close to being fully released, and I am worried that it will stay that way. These screenshots I took were in the same superflat world in a small tunnel I dug down. I wonder if it's the sunlight interfering with the torch lighting that makes the transition so abrupt.
Both of these are bugged, though the one of the right is much worse, almost as if smooth lighting isn't enabled but you shouldn't get that effect if it is disabled; it should look like this, as fixed by myself years ago, shame on Mojang as somebody actually gave the fixes, which I used, on this bug report (this isn't all that is needed but it will fix most of the most obvious cases):
Here is another example:
In vanilla you get this, in addition to a sharp transition from light level 1-0 since the game doesn't properly handle this case (it smooths lighting by mixing the light levels in the 4 blocks surrounding each vertex; vanilla sets anything with a light level of 0 to the same level as above the face being rendered, while I fix this by completely ignoring points inside a solid block, but not otherwise 0, and only mixing the remaining points):
There's also a semi-related bug; ambient occlusion has a bias towards certain directions (this is the darkening seen in corners, which is part of the "smooth lighting" setting but isn't actually the same thing), when it should look like this:
There should also be no difference between sky light, block light, or a combination since the game handles them the same way when calculating the brightness of each vertex (it combines them into a single value so it only need to do the calculations, which are rather expensive, once).