Driven properties are serialized as 0; otherwise they could constantly change, for example based on current Game view size. When a Prefab with driven properties is saved or applied, it will thus have zeroes for driven values. When this Prefab is instantiated, the values will normally be driven again, and so get their proper driven values. But if they are no longer driven, they will be zero. In the case here, a CanvasScaler component was driving the localScale when the Prefab was saved (it was an outermost Canvas) but the Prefab is then instantiated as a sub-canvas under another Canvas, where CanvasScaler components have no effect (as shown in a warning in the Insector). Thus the scale is not driven, and remains zero as it is on disk. It's advisable to not use CanvasScaler components on Prefabs that are meant to be used under a different Canvas.

Note that it's tricky to compare the behavior to earlier version of Unity. Unity 2018.2 had a bug where driven properties were not shown as such in the Inspector, and certain previous versions of Unity had driven properties temporarily work in a different way, which however caused a lot of regressions so that it was later reverted. Furthermore, Versions prior to 2018.3 didn't have Prefab Mode, and inspecting a Prefab in Prefab mode is not the same as inspecting the asset in the Inspector in earlier versions, since Prefab Mode causes driven properties to be actively driven, while inspecting a Prefab Asset in the Inspector in 2018.2 and earlier does not.