I've just answered a similar question. This is something you need to take up in the indoor plant forum. Possibly the reason may be that moisture levels are not maintained. I think this plant is sensitive to low sunlight and also that it;s soil must not be allowed to dry out completely. Irregular watering may have stressed it out. Anyway, please contact the indoor plant forum and ask for people who are familiar with your plant.

I made a mistake thinking there was an indoor plant forum, but what I was referring to was the houseplant forum. However in doing a search on myrtle, I came up with zero. No one has requested help on plant care. I also did another search on google. Turns out there is just not that much information on the subject.

I suggest you contact Humber Nurseries or Sheridan nurseries for advise. They may be able to assist with this plant.