If it takes longer than the configured time to gather information from all the tablet servers, the thread pool stops and processing continues with whatever has been collected. Code is https://github.com/apache/accumulo/blob/1.8/server/master/src/main/java/org/apache/accumulo/master/Master.java#L1120, default timeout is 6s. Does not appear to be an issue prior to 1.8.
Best case, this was really confusing. The monitor page would have 30 tservers, then 5 tservers. Didn't really see any other negative effects, no migrations and no balancing appeared to be affected. Worse case though, I missed something and the master is making decisions based on incomplete information.
Dave Marion please add more info if needed.