Auto Bed Levelling - Printer not detecting, but sensor is triggering
Perhaps the culprit is a pull-up resistor on the board. Normally, endstops on 3D printers use the microcontroller's internal pullups. These have a resistance of around 50kΩ, which is far too high to be a problem. However, if lower value pull up resistors are used on your main board, this could cause a problem.

The resistor of your voltage divider form, when the output of the sensor is low, a parallel pair of resistors to ground, with an effective resistance equal to 1/(1/10+1/15) = 6kΩ. If there was (let's say) a 4.7k pull-up resistor on the board, you'd expect to see around 2.8V on the output (because the pull-up resistor, together with the two resistors of your voltage divider, forms another voltage divider).

I don't have the Anet A8 main board myself, but on pictures I do see a set of 6 resistors suspiciously close to the thermistor and endstop connectors.

You could verify my suspicions by unplugging the endstop, powering down the electronics and then measuring the resistance between the endstop signal and 5V pins.

Possible solutions:


Desolder the offending resistor. This is pretty easy with SMD parts: you just alternate between heating up both sides until it slides off.
Use a diode in place of a voltage divider. Anode goes to the endstop connector, cathode to the signal of the probe. This prevents the high voltage of the probe from being seen by the electronics, while allowing the probe to drain the current from the pull-up resistor.


With this last solution, make sure the reverse leakage current of the diode is not too high. If it has a reverse current of (let's say) 50uA, then 50uA flowing through the (supposed 4.7k) pull-up to ground would raise the voltage at the signal pin to 5.002V. This is unlikely to be a problem, but with higher value resistors or higher leakage you'd see the voltage raise higher above 5V (which the microcontroller won't like).