Modem Firmware Blank after Sign In iCloud Account

Hello apple

I want to complain about what happened to my iPhone X

I use an iPhone X 64 Gb with model number MQAC2ZP/A.

This iPhone was normal as usual, but in 21st March 2020, my iPhone suddenly lost signal, and keep searching signal, control center being lag, open setting being lag, and open submenu Cellular automatically crash.

I was change my sim card, reset all setting, reset network setting, erase all content and setting still doesn’t have effect. It stil searching. And then I open sub menu general > setting > and in modem firmware was blank.

And then I restore my iPhone to iOS 13.3.1, and voila its back.

but when I sign in my iCloud back, it does happen again. Keep searching, and modem firmware is blank.

but when I sign out my iCloud, its become normal again.

Please apple what happened to my iPhone and my iCloud