Rpi can not read from console tty since kernel 4.14

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I have an app on my headless RPi which runs from boot via a systemd service file with StandardInput=tty. It has been running like this for 18 months and I have updated my system very frequently. However, since the kernel update:

2018-02-13 07:02 linux-raspberrypi 4.9.80-1 -> 4.14.18-1

The console input is not attaching to the app. I downgraded the kernel back to 4.9.80-1 (and a few others before that) where it works. I recreated the same issue and downgraded to fix on another RPi also.

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OK, I have fixed this problem. Previously, I have always removed the console=ttyAMA0 and console=tty1 options from /boot/cmdline.txt and this has always worked. It seems now I must keep the console=tty1 option there because adding that back and using the 4.14 kernel works. Possibly the 4.14 kernel is somehow automatically assigning console=ttyAMA0 if there is no explicit console option, whereas previous kernels did not.