I do not know if this was a conscious choice or if it's a bug. Either way IMHO it should be changed.

When rolling over one of the data lines in a sim plot, a tooltip appears showing details. For booster stages the name of the stage is shown at the top of the tooltip. For the sustainer stage (including single-stage rockets) the name of the stage is not given.

I believe the stage name should be shown for all stages in multi-stage rockets. It's debatable whether it should be shown for single-stage rockets, but I'd be inclined to include it.