Actual:

Creating subscriptions with queries having unicode parameters causes a server error:
query from client: "{\"Product\":\"(stringQueryField BEGINSWITH B64\\\"2KzZhdi52Kpz\\\" )\"}" could not be parsed: unsupported query for table "Product": can't use BEGINSWITH operator on a non-string constant (ProtocolErrorCode=300)
Generated query is:

Query:{
  "Product": "(stringQueryField BEGINSWITH B64\"2KzZhdi52Kpz\" )"
} 

This is because the base64 encoding in serializer.cpp: https://github.com/realm/realm-core/blob/master/src/realm/util/serializer.cpp#L116

Expected:

The query should be generated without base64 encoding like this:

Query:
{
  "Product": "(stringQueryField BEGINSWITH \"جمعتs\" )"
} 

Note: The issue is reproduced for queries with BEGINSWITH and CONTAINS, but it should be fixed for all the queries.