build: changing the package flags on command line has no impact on subsequent rebuilds

I have the following conditional in my cabal file:

```
if flag(native)
ghc-options: -hide-package "unicode-transforms"
-package "unicode-transforms (Data.Text.NormalizeNative as Data.Text.Normalize)"

```

The flag works correctly (enabled/disabled) on a fresh clean build. But when I rebuild with a flag value which is different from the previous build then the project is not rebuilt with the changed flag. Here is the output after a changed flag:

```
cueball:/vol/hosts/cueball/workspace/projects/unicode-transforms$ stack build --flag unicode-transforms:native :bench
unicode-transforms-0.1.0.0: unregistering (flags changed from ["-f-native"] to ["-fnative"])
unicode-transforms-0.1.0.0: configure (lib + exe + bench)
Configuring unicode-transforms-0.1.0.0...
Warning: 'ghc-options: -hide-package' is never needed. Cabal hides all
packages.
unicode-transforms-0.1.0.0: build (lib + exe + bench)
Preprocessing library unicode-transforms-0.1.0.0...
In-place registering unicode-transforms-0.1.0.0...
Preprocessing executable 'ucd2haskell' for unicode-transforms-0.1.0.0...
Preprocessing benchmark 'bench' for unicode-transforms-0.1.0.0...
unicode-transforms-0.1.0.0: copy/register
Installing library in
/vol/hosts/cueball/workspace/projects/unicode-transforms/.stack-work/install/x86_64-linux/lts-5.8/7.10.3/lib/x86_64-linux-ghc-7.10.3/unicode-transforms-0.1.0.0-0Q21KoLEyBN0p0f5WJB5QO
Installing executable(s) in
/vol/hosts/cueball/workspace/projects/unicode-transforms/.stack-work/install/x86_64-linux/lts-5.8/7.10.3/bin
Registering unicode-transforms-0.1.0.0...
```

Also, note that it says `Warning: 'ghc-options: -hide-package' is never needed` but in my case it is needed because I am hiding the package and then exporting it with modules renamed. The renaming conflicts with an existing module with the same name in the same package if I do not hide it.

Is stack somehow thinking that none of the compilation flags have actually changed? And therefore there is no need to recompile?