The + key does not specifically edit Exposure or Vibrance. The - / + keys are shortcuts that affect whatever setting is currently active for those shortcuts, so for you, that means for many years Exposure had focus for the - / + keys, but somehow this has recently changed.
The setting affected by - / + keys can be changed in two ways. One is to click on the setting name’s text, and the other is to press the , / . keys (intended to be read < / >, in other words previous/next) until it cycles around to Exposure. If either of these was done unintentionally, it could have switched the current setting away from Exposure and to Vibrance.
The quickest solution is to set keyboard focus back to Exposure by clicking the Exposure setting name text next to the slider. You’ll notice a brief overlay message that lets you know it took focus.
As for where this is documented, it’s pretty obscure, so definitely not easy to find by searching. The only reason I could find it is because I already knew how the shortcuts worked. These are listed on the Adobe Help web page Keyboard Shortcuts, under the section Keys for Working in the Develop Module.
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