BLE different MTU for different implementations

Question

I have tried different implementations of BLE connection on Android. One with RxAndroidBle and another one with simple Android API. I used RxAndroidBle example app for testing. I connect to the same peripheral with the same service and characteristic. Though when I read or get notifications from it in the case of RxAndroidBle I receive 512 bytes and in the case of Android API - just 20. I try to request MTU 512 but onMtuChanged is never called and I still receive 20. Do I miss something?