I upgrade a 7.9.0 redundant gateway to 7.9.1-RC1,
the system tag Gateway\Redundancy\IsActive is broken.
When I browse in designer, I have : Gateway\Redundancy\IsActive\Null ???
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Always this issue with 7.9.1.
Scan class driven by tag [System]Gateway\Redundancy\IsActive seem not working anymore ???
This issue seem occur after the first gateway restart… 
The issue seems to be Expression Tag (or Driven scan class) with reference to a tag path like [System]…
When you reference to [System]Gateway\Redundancy\IsActive in an expression tag, this tag ([System]Gateway\Redundancy\IsActive) is broken in the designer, after a gateway restart !
This issue might be fixed in the recent production version of Ignition 7.9.1. Just saying it might be fixed.
Best,
Hi Nick,
Issue is still there in Version: 7.9.1.20170125-1117 
This has been fixed for 7.9.2.
Hope a 7.9.2 beta or release candidate will available soon to check this fix !