I think this is a pretty mundane kind of case—it happens all the time. (There are other kinds of cases with the relevant feature too—imagine a case where one does the wrong thing by refraining from asserting. One may—indeed, ought—to assert, but doesn't.) But I also think it's a counterexample to Rachel McKinnon's 'Supportive Reasons Norm', which she suggests is the central norm governing assertion.
Here is the Supportive Reasons Norm (given on p. 52 of Rachel's recent assertion book):
One may assert that p only ifMy case of Helen is a counterexample because Helen may assert that sloths urinate only once a week, but fails to satisfy condition (iii), since she doesn't make the relevant assertion at all, let alone for a particular reason. In general, that condition will ensure that the permissibility of the assertion entails that the assertion is made. Since it's not true we're only permitted to assert the things we do assert, I don't think condition (iii) is part of a proper characterization of what one may assert. (It is much easier to think something like it may have a role to play in a characterization of when a given assertion is a proper one. Perhaps that's what Rachel had in mind.)
(i) One has supportive reasons for p,
(ii) The relevant conventional and pragmatic elements of the context are present, and
(iii) One asserts that p at least in part because the assertion that p satisfies (i) and (ii).