Hi 443683123,
Are you using the devkit or custom board for Xavier NX?
You are saying that the “drone” so that I suppose you are using the custom carrier board rather than the devkit.
Yes, it seems caused from the serial data sending from your MCU.
This patch should help for your case to disable the feature to enter into UEFI menu.
Please flash your board through command to apply the change.
You can also refer to Q: There is a wait in UEFI stage. How to disable it to accelerate system booting? to remove the 5s auto boot time in UEFI.