There is a big issue present in both versions where the volume transmission looks distorted, this is caused by you using Shade Smooth on a low poly mesh with a transmission shader. Low poly objects usually produce issues like these unless Auto Smooth is used and the settings are tweaked to get the right results. You can find the setting for Auto Smooth here:

The reason why this rendering artifact doesn't occur in the final render is due to the fact you have a sub-division surface modifier that increases the geometry which is only visible in the final render.

As for the other issue, the white spot in the middle of the cylinder, that appears to be another issue and with testing I found that this issue is present prior to the Cycles-X merge. So I will bisect for the cause.

Yes, the artifact is connected to the Geometry Offset. This feature lifts starting point of the reflection ray above the geometry to prevent shadow terminator artifact T68920. Sometimes it overshoots and the starting point ends up in a bad place. Because of this you have to be very careful with smooth normals: the bigger the polygon and angles with its adjacent polygons - the bigger is the zone where you shouldn't place other geometry.

@Alaska (Alaska) is correct: when you final-render the scene, subdivisions make the issue go away because polygons and angles become much smaller. But there's also a problem with the object itself in the first place: you have a bevel modifier that creates this ill geometry. In most cases (yours included) the correct way to use the bevel modifier is to activate Harden normals in the bevel settings and to enable Auto smooth in the object data properties. In fact, I think that these options should be activated by default.

Thanks to this report I also noticed that the Geometry Offset now depends on the object scale. I was deliberately writing the terminator fix to correctly handle object transformations and had tests for this, but probably I have to revisit it.