I chose Close memory from the Memory menu to close a memory that I had joined to Google Translate, Microsoft Translator and Yandex.
Not only the memory was closed: the whole tab was closed, including the MT engines.
I wouldn't be surprised if the fixing of this issue would also solve the unjoining of the three selected MT engines when a new project is created, with another memory. So, I'd be very happy with a fix ;).
(I feel a kind of stupid that I always have to join these engines, every time when I create a new project, with a different memory. I hate routine tasks.)