MySql binlog file getting truncated/shrinked during replication

There is a master-slave setup of a mysql db. The replication was going on fine until pos y when an error was thrown Client requested master to start replication from position > file size. I tried to restart replication from an earlier position but it failed. I checked for binlog file size and it seems the size is currently x (with x <<< y). However, I do have logs for replication from pos x to pos y. I don't have access or knowledge of any activity on the db. Didn't find any relevant article where binlog file was truncated to a smaller size. I want to understand what all happening on the db could shrink a particular binlog file? Does restart or failover could lead to such a scenario?