In case you're wondering what HDR does, when you take a picture with the setting on, three different versions of each picture you snap are produced inside your phone. Each of the three are at different exposures. Your phone will do the heavy lifting as it puts the three exposures together to form one better looking photo.
HTC says that it did not remove the feature and is blaming the disappearance of it on a bug that was part of the software update. But HTC says that it is aware of the issue and that it is working on a fix. Hopefully, for T-Mobie customers with the HTC One, HDR will reappear soon.
UPDATE: T-Mobile and HTC have already started sending out the update that returns the HDR feature to the HTC One camera. Kudos to the pair for their quick work in resolving this issue.