Currently, there seems to be some issue with the expression used in a Math Converter and there being a mismatch between the value shown on-Stage, the source Value, and the expected mathematical result of the Converter's Expression.

See attached video.

Repro Steps:

1) Open the Coriolis Sample from the InternalProjects GitHub.

2) Introduce a TextBlock to the Stage, and configure it as to make the writing visible.

3) Bind the TextBlock's 'Text' Property to any of the 'Metrics' in the current Data Context.
Observe how the number in the yellow-bordered 'Text' Property field matches the value shown on the Page. (as expected)

4) Add a MathConverter to the Binding, leaving its expression as the default "{0}".
Observe how the number in the yellow-bordered 'Text' Property field matches the value shown on the Page. (as expected)

5) Change the Expression from "{0}" to {0}+{0}"
Observe how the number displayed on-Stage is double the number shown in the yellow-bordered 'Text' Property field. (as expected)

6) Without changing the Expression, change the Data Path, to bind the TextBlock to a different 'Metric.'
Observe how the number in the yellow-bordered 'Text' Property field matches the number shown on the Page, when the number shown on-Stage should have displayed a number double the value of the number in the 'Text' Property field.

7) Swap being 'metrics' and observe the numbers are failing similarly.

8) Change the Expression from "{0}+{0}" to "{0}+1".
Observe how the number on-Stage is showing the correct value relative to the Data Value, but now the number in the yellow-bordered Property field is showing a number that corresponds to "{0}+{0}" instead of "{0}+1".

Expected Results: The on-Stage value always corresponds to the value of the Data combined with the Expression.
Expected Results: Unsure if the value in the Property should always match the 'Source' Data Value (without combination the Expression), or whether the Property Value should display the Expression's result.

In any case, there appear to be some syncing issues between the three locations in which values are displayed.