I discovered that OpenHab was enabled at the same time even if I removed the gateway from its things (just wanted to give it a try), so I just unistalled it and the problem disappeared.
For a while...
I was using MYSController for real time debugging of my sensors, but without enablig the "RX only" mode.
Once enabled the RX only mode, the problem disappeared completely.
So, there's nothing wrong with MyController, but rather a weird scenario caused by me.
Enabling ACK on both sensors and Mycontroller I could figure out what was going on.