I agree. I’m not sure whether it should be classified as a bug or not.
This is clear from the manual. It is explicitly mentioned in a note:
Note
Labels that have the same name will connect, regardless of the label type, if they are in the same sheet.
2. Global buses
are not mentioned at all in the schematic editor manual, neither is the the alias list mentioned in combination with global labels, so the documentation could be updated here. I guess this combination has not even been tested or used at all. Turning the whole alias list into global labels when the alias name is used as a global label is logical from a software point of view, but it surprised me when I first saw it here.
3. Bus aliases are defined in sheets, so adding different sheets can change behavior
Apart from promoting the whole alias list to global labels on a sheet when the alias name is used as a global label, I am not aware of other changed behavior. Any examples?
Another item that can use improvement in the documentation, is that “bus connectors” are optional. In the example below, “wheel” is in the alias list and it connects to the bus normally. However, pins 2 and 6 are also connected to each other because they both have the same label. It would be easy to make the mistake to assume that pin 6 will not be a part of the bus.
And the same when the bus itself is named:
In old KiCad versions (V4, V5) the blue bus lines were purely eye candy and it was mentioned clearly in the manual that connections depended on the labels to match. This section is no longer in the manual of the schematic editor.