I recommend making honesty and candour a general part of your dealings with others, so I would recommend being open to disclosing limitations and obstacles in this type of situation. But before you even get to that part, agonising over whether you should or should not disclose this information is jumping forward a step. The more important preliminary step is to ask yourself why you think you will succeed in a PhD candidature when you experienced burnout even over a substantially shorter Masters program. Presumably you have thought about this and have a view about it, so either you have some good reason to believe that you will do better this time, or you are likely making a bad decision for yourself by pursuing a PhD candidature at this time.
Start by engaging in this type of introspection and have a think about what, if anything, has changed since you did your Masters program. What caused the burnout in your Masters program? How did that affect your life? What, if anything, has changed since then that would make you more resilient to this type of burnout (or less susceptible to its cause)? If you were to eschew a PhD candidature right now, what are your other options? Is a PhD candidature a good pathway for you right now relative to these other options? Would you benefit from some other opportunity at present (e.g., experience in the workforce, etc.)? If you can think sensibly about these questions and make the right decision for yourself, that should significantly reduce your anxieties about explaining your position to others. Often anxiety over disclosing true information in such cases stems from a lack of confidence that the problem raised by that disclosure is actually solved.
Now, assuming you decide to go through with your application and you get to the interview, what can you say if the matter is raised? Firstly, you can point out that you finished your Masters program successfully, albeit in a longer timeframe than initially planned. This shows that you can experience an adverse circumstance and push through it to successful completion. Secondly, you should have done some introspection on this experience and learned from it, so you should now have a set of methods (perhaps including taking longer) that will allow you to get through a PhD candidature even if you experience burnout again. Once you are able to explain your pathway adequately to yourself, you should be able to explain it to others.