Currently, it appears that at around 70% zoom, TextBlock Text Decorations don't render. They seem to only appear if above 70% zoom.
See attached video.

Repro Steps:

1) From within the MiniShowcases Project, open the DataGeneration.xaml sample
2) Zoom in and out of the Stage, keeping an eye on the underlines present under the Data-Generated words in the Mayor's Module in the upper-right.

Observe how at certain zoom levels, the underlines disappear/appear.

Expected Results: TextDecorations always render regardless of zoom level.