The FKM plugin has a mechanism to auto-detect if a log file has "rolled", i.e. been replaced with a new copy. When this happens the plugin starts reading the beginning of what it consider the new file. This is not influenced by the Rewind setting in anyway.
For this reason, when you are testing FKM, you should only append lines using basic tools like the UNIX shell's '>>' redirection operator and you should avoid using any text editors that may rewrite the entire file. This is most often something like vi and it's derivatives. They create a temporary file with any changes you have made and when writing out those changes it simply moves the temporary file over the original. This triggers FKM to see a new file - because it has a different inode number on UNIX/Linux, but similarly for Windows systems.
If you do use a method that recreates the file then FKM will report all the matches in the file from the start which may not be what you expect and may cause confusion at the result.