The normals of the faces were facing inwards.
Solution: "Recalculate Outside" by selecting all faces of the object in edit mode with A and then pressing Shift+N.
Many operations while modelling can accidentally cause messed up normals, especially if one uses shortcuts and presses the wrong keys(combinations). As face orientation is not very obvious if not explicitly displayed, it can be missed til later times when the shading suddenly does not work as expected. With the cause and effect delayed by some time it usually does not come to mind where the problem originated. That's why it's good practice to check the "Face Orientation" in between from time to time, preventing confusion in the long run.
Further images are with the option activated
Meshes consisting of vertices, edges and faces have an inside and outside, which normally is the way it is seen from, outside. Thus those faces facing outside are the ones representing the surface of the object and are used to show the materials and such. The other side of the faces would be the inside of the object, where normally you'd not look at them from.
If you'd build an inside of sorts, you would build a thickness, resulting of a layer of faces that generate a room between them, the outer layer facing outside, the inner layer facing inside. Again, you would only see the outside faces of the object. Here a demonstration (cut off to show the red inwards and blue outwards facing normals.
That's because inside/backsides of faces do not get calculated the same way for visualization as in render or any form of shading.
Also affected are subdivision modifiers, and they generate a big part of the effects leading to confusion. Here a simple example with one face wrong oriented on a subdivided cube:
The modifier seems to completely ignore the wrong oriented face
That's why when two faces beside each other have different normal orientation, the shading will generate a break between them depending on the materials used that can become even more obvious. Glass is one of those extremes. Using the "Face Orientation" option is the easiest way to check and find problems as it displays the orientation of faces blue for outside and red for inside as you look at the outside of the object, all should be blue.
While most the time Blender manages to correct normals if you select all elements (vertices/edges/faces) in edit mode with A and press Shift + N, it may be necessary to manually select the faces involved and use the Alt+N Normals menu and select Flip to correct the orientation.