To reproduce:
1. Open attached project
2. Build to Samsung device with S-Pen
3. Use the Pen on the mobile screen

Actual result: The pen does not work. Getting Mouse0 in Input.GetKey only at first draw call. Then you don't receive anything (Input.GetKey return false all the time you press S-Pen)
Expected result: The pen should work. When you press and hold S-Pen should get Mouse0 button pressed all the time while pressing S-Pen (Input.GetKey)

Reproduced with: 2017.4.33f1; 2018.4.9f1; 2020.1.0a4
Not reproduced with: 2017.4.32f1; 2018.4.8f1

Tested with:
VLNQA00219, Samsung Galaxy Note9 (SM-N960U), Android 8.1.0, CPU: Snapdragon 845 SDM845, GPU: Adreno (TM) 630
VLNQA00128, Samsung Galaxy Note8 (SM-N950F), Android 9, CPU: Exynos 9 Octa 8895, GPU: Mali-G71
VLNQA00015, Samsung Galaxy Note8 (SM-N950W), Android 8.0.0, CPU: Snapdragon 835 MSM8998, GPU: Adreno (TM) 540
VLNQA00028, Samsung Galaxy Note4 (SM-N910F), Android 6.0.1, CPU: Snapdragon 805 APQ8084, GPU: Adreno (TM) 420

Note: Logcat attached.