Why Does the Input Voltage Drop When a Linear Regulator is Added as Load
The regulator needs to have a capacitor, around 0.33uF nearby between input and ground. Also a small capacitor 0.1uF between output and ground is good practice. The regulator could be unstable without it, and without a little bit of load on the output. That may explain the current draw, and resulting voltage drop across R22.  Speaking of R22, normally it is a bad idea to have a high resistance on the power feed to a regulator. Also, it is worth noting that the regulator will only work if the voltage drop from input to output is at least 2 volts. So if you need 5 V out, the input pin on the regulator must be at least 7 V.