Back in November of last year, AT&T announced a new feature called 'Stream Saver.' This would force streaming videos to be limited to around 480p - but still count towards your data usage. AT&T made it clear that the feature would be enabled by default when it arrived in "early 2017," and now it appears to be rolling out to customers.
Thankfully, it's not too difficult to turn off. Simply visit the 'Manage my data usage' page on your AT&T account, and you should see a switch for Stream Saver on each of your devices. The company says that toggling it could take around five minutes and requires a reboot to take effect.
AT&T enabling this by default seems scummy to me (how many people are going to miss the above notification and wonder why streaming quality is so low?), but I can see it being useful in a few cases. For example, if you're a parent and your child really likes streaming Care Bears on Netflix, you can turn Stream Saver on to make sure they don't blow through your data allotment.
Let us know in the comments if Stream Saver has been enabled for your account.