Depends on what you mean by “undecidable”. There may be situations in which it's hard in practice to decide whether it's better to do A or to do B, sure, but in principle either A is better, B is better, or the choice doesn't matter.

That one thing a couple years ago qualifies.

But unless you get into self-referencing moral problems, no. I can't think of one off the top of my head, but I suspect that you can find ones among decisions that affect your decision algorithm and decisions where your decision-making algorithm affects the possible outcomes. Probably like Newcomb's problem, only twistier.

(Warning: this may be basilisk territory.)

How are you defining morality? If we use a shorthand definition that morality is a system that guides proper human action, then any "true moral dilemmas" would be a critique of whatever moral system failed to provide an answer, not proof that "true moral dilemmas" existed.

We have to make some choice. If a moral system stops giving us any useful guidance when faced with sufficiently difficult problems, that simply indicates a problem with the moral system.

ETA: For example, if I have completely strict sense of ethics based upon deontology, I may feel an absolute prohibition on lying and an absolute prohibition on allowing humans to die. That would create an moral dilemma for that system in the classical case of Nazis seeking Jews that I'm hiding in my house. So I'd have to switch to a different ethical system. If I switched to a system of deontology with a value hierarchy, I could conclude that human life has a higher value than telling the truth to governmental authorities under the circumstances and then decide to lie, solving the dilemma.

I strongly suspect that all true moral dilemmas are artifacts of the limitations of distinct moral systems, not morality per se. Since I am skeptical of moral realism, that is all the more the case; if morality can't tell us how to act, it's literally useless. We have to have some process for deciding on our actions.