You're right in that no one outside of corporate staff or those on a need to know basis has any solid information of the nature of their deal. The only thing we can surmise from the past is that Fox had a form of use it or lose it (my term, not a legal one!) I think the evidence is pretty clear when things seemed to really go sour when Fox put the Fantastic Four into production in 2014. Rise of the Silver Surfer was released in 2007. Some reports I've read over the years say that Fox could keep the rights to the Fantastic Four as long as they put something in production every 7 years. Sometimes I've read 8 years. I guess it depends on whether the agreement goes by when production dates or release dates. But since Ike Perlmutter allegedly went on his rampage in the fall of 2014, it would seem to indicate that Fox's decision to release another Fantastic Four movie to set the clock back to zero again is the likely trigger.
The rights have a strange history because they were originally bought by a European company called Constantin Films back in 1986 and at one point Chris Columbus was attached to the project. Columbus says even though his name appeared on the two films, he was actually fired but they kept his name on the credits. By the mid 1990s, there was still no Fantastic Four movie and then we got the rushed, never to be release Roger Corman movie in 1994 to keep the rights. So that leads one to believe that there was a cut off date in place or they would not have rushed something into production but not release it.
In this
interview clip Chris Columbus says his ideas were rejected. Constantin Films then made the deal for Fox to produce the films. It's still a bit murky on how Fox ended up with it and Constantin Film is apparently no longer part of this purchase process. Fox provided the resources for the film production and the distribution. Or maybe Disney made some settlement with Constantin Film. There's been very little mention of them in this process so maybe the picture will become more clear when the deal goes through.