I just realized why my plays keep re-running roles when running a certain role.
It is because I have dutifully replicated the same parameters used in the play for each role in the dependencies list, which includes the tags parameter.

So when I call the play with --tags R, the R role (which is tagged eponymously) triggers, but also any role in the play which lists R in its dependencies, because I tagged the dependencies!

The solution is obvious and I am embarrassed it has taking me so long to realize: do not tag roles in the dependencies list!