An ESP only has 1 ADC pin and on an ESP-01 it is not exposed.
If you use D8 on an ESP-01 you are connecting to a pin (GPIO 15) which is not exposed.
On an ESP8266 some of the pins need to have a certain state at boot.
GPIO 0, 1 & 2 should not be pulled LOW. GPIO 15 should not be pulled HIGH (the latter is what you may have run into)
I anyway don't think it is the proper way of connecting to a 12v source on a 3.3v MCU. I suggest you use an Opto-coupler to separate and make sure that the inoput never exceeds 3.3v