Yes, this is a bug. Typing `Q.multiplicity??`

we see that it calls `X.multiplicity(Q)`

and `X.multiplicity??`

shows that it changes the monomial ordering to `negdegrevlex`

and then tries to use the interface to Singular. However the interface to Singular is broken for *univariate* polynomial rings with "local" monomial ordering (it always uses the global ordering instead); I reported this as trac ticket #27479.

As a temporary workaround I guess you can add a variable and set it to zero:

```
sage: A2.<x,y> = AffineSpace(QQ,2)
sage: X = A2.subscheme([x^1789+x,y])
sage: Q = X([0,0])
sage: Q.multiplicity()
1
```