If you specify @GET("foobar?a=5"), then any @Query("b") must be appended using &, producing something like foobar?a=5&b=7.
If you specify @GET("foobar"), then the first @Query must be appended using ?, producing something like foobar?b=7.
That’s how Retrofit works.
When you specify @GET("foobar?"), Retrofit thinks you already gave some query parameter, and appends more query parameters using &.
Remove the ?, and you will get the desired result.