There is one thing we know for sure - Bruce Willis is no longer working on Woody Allen's currently untitled period piece. The reasoning behind his departure, however, depends rather drastically on who you ask.
According to both Woody Allen and Bruce Willis' people, Willis left the production due to scheduling conflicts emerging from his up-coming performance in the Broadway adaptation of Stephen King's Misery. But, if you ask pretty much anyone else, they'll probably tell you he was fired.
Deadline's Mike Flemming Jr. was the first movie journalist to debut the 'exclusive,' stating "Willis’ plan to work with Allen was hobbled by his other plan to take the Broadway stage in the adaptation of the Stephen King bestseller and movie classic Misery." The Wrap's Jeff Sneider disagrees, pointing out that Willis was seen on the set of Allen's project the day of the announcement of his supposed scheduling conflicts.
Of course, scheduling conflicts aren't usually something you suddenly remember half-way through a day of filming, which adds credence to the suggestion he was fired. There is also further fuel for the firing rumor, with one reply on Sneider's tweet claiming he was dropped because he could not remember the lines from Allen's script, resulting in frustration for the remaining cast. When you throw in the fact we've seen Willis on set - and in costume - beside co-stars Kristen Stewart and Jesse Eisenberg, it seems likely 'scheduling conflicts' actually had little to do with Willis' divorce from the project.
If this is indeed the case, it wouldn't be terribly surprising. Both Willis and Allen have previous experiences in this area. Willis has famously rubbed some directors, namely Kevin Smith, up the wrong way with his behaviour on set, while Allen has also fired actors, even big named ones, early on in a movie's production - especially if they're having issues with the script. For example, he famously dismissed Michael Keaton from the set of The Purple Rose of Cairo three weeks into production.
With this in mind, it's looking likely 'scheduling conflicts' is simply an excuse which allows both parties in a possibly unsavory situation to save face.