It is not expected that it uses so much memory. It is possible that Syncthing Tray or one of its dependencies leak resources. One could use valgrind to investigate this potential problem.
Note that memory freed by an application is not necessarily immediately reflected by the OS memory statistics. I once had an application with a huge in-memory db which could regenerate all the data and then throw away the old data. Throwing away the old data was not fully reflected in the OS memory statistics. However, even after weeks it never exceeded a certain memory usage after all.