I do believe Celebrity that the problems are directly related to the Mermaid propulsion system. With only two exceptions, all
ships equipped with those systems have suffered from major problems with their podded propulsion at least once. With two exceptions, all
of them had to be drydocked in order to work on the pods.
You are correct about the stability problem with the "Millennium", which had to be corrected. However, "Infinity" and the two other ships in the class were modified even before Celebrity accepted delivery.
Any stability problems of that kind do not arise all of a sudden. Therefore, I do not at all believe that this is the reason behind "Infinity's" drydocking. If there were significant problems, Celebrity would have made sure to correct them during "Infinity's" last drydocking.