testfor @e {Rotation:[0.0f,0.0f]}
entitydata @e {Rotation:[90.0f,0.0f]}
attempt to run these and mess with testing for the rotation of the entity and you will see some strange behaviors...
Sorry I do not post bugs a lot, logging into this site is a pain; ill attempt to post all of them more often
This may be the same case that the powered NBT tag in command blocks had where if you change it the outcome may vary. (Not supported/but it is functioning as intended)
I understand you can testfor rotation in the selector, but was wondering why you can set it to 90, but it not be able to return the test results properly for being at the angle 90.... and if it was supose to represent rotating a specfic amount than why not have it reset to 0 each time, or contain no value at all after the tag is edited (Edit= follow through with rotation then if tested upon you either wouldnt see the rotation or perhaps it would reset to 0)
Again all speculation on to why it is functioning this way... Thank you for the help!
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hmm well read the end portion and that should be some elaboration.... It may or may NOT be a bug... may be intended feature...
You can test for a entities rotation inside of a selector and then act upon that data... but when you do the same thing with the NBT tag you get unexpected results (varies.....) like you cannot change it then test for it and expect it to return true...
I try to explain it in my original post....
Example:
I am a Zombie with the Rotation NBT tag set to 90.0f,0.0f and I test for that but it wont return true...
Um but if you edit the NBt anyways regardless of the tests the tests vary, you can get it to work to return true if you mess with it... but please use the two commands I list and watch to see what I mean by either intended features or bug.....
I still don't understand. I just gave a rotation to a creeper, tested for mobs with that rotation and it found the creeper. Once it started moving it didn't find it anymore.
okay so summon an armorstand with NoAI
Then try the rotation tests and you will see it manages to not work
Sorry !
EDIt: lol armorstand noai, haha.... um ignore that lol just armorstand
Hook up a clock to testfor a specific rotation of an armorstand, give it a few entitydata changes to its Rotation NBT tag and it does not retain its value.. The output varies, but it is very easy to replicate....
I am having another administrator attempt to explain in his works instead.....
The Bug you listed sounds similar yes, looking into it now
Seems like this bug has been out for awhile I supose (It effect mutliple NBT Tag depending on what NBT has been set)
We ran into similar issues while using a combo of levitation, rotation, motion, nogravity which leads to some NBT edit not taking effect...
We will continue to look into it...

@unknown please modify the description to precisely explain what you did and what you expected. In 16w36a all command provided in this comment (replaced with new entity ids) work.
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You'll have to elaborate - what strange behavior exactly are you talking about? Please also provide detailed steps to reproduce the issue.