I did an experiment when I was review-testing Weather Radar & the first iteration of the 767.  I set up some very high-topped clouds in FSX -- I think they were 56,000 foot monsters -- and enabled ice, and turbulence.  Pretty much the same conditions as the Air France Airbus faced.

The weather radar performed beautifully, but the point was not to avoid the bad weather in this instance.  Into the maw flew I, and quickly lost control in the same manner.  So I suspect that it is indeed weather-related, although the two times with REX it was in clear air, not clouds.

I posted on the REX site without response.  Right now I am flying from Bahrain to London with REX and so far so good, although I turned off the weather engine after the tell-tale difficulties reaching cruise altitude.  Now I'm stick with a 108 kt. headwind that I hadn't planned for, based on the charts, so I'm going to have to do something about that, I guess.

I love how REX looks, but I may use the textures with the FSX weather engine for long hauls.  I am beginning to have my suspicions about its suitability for those.