Hi Ronan, We have seen this, both with the printer being off for a while and if you try to connect to it while it is out of range or offline. We only see it with Android 6.0 and above and only on some devices. This behavior is controlled by the Android OS so there is only so much we can do from the printer. That being said, we are gathering information about this issue so that we can properly find fixes. Can you share the models and OS versions of your Android devices? If you found you don't have the issue on some devices, what are those models and versions?
You might also consider switching to BTLE on the new ZQ520 printer. It might be slower but we have not seen this issue come up using it, as it doesn't require pairing. We have a separate library for Android BTLE in the SDK.