Arduino Vin pin does not work, but Arduino 5v pin power works
The Vin pin on the Arduino Uno board is connected to the input of the 5V onboard voltage regulator of the Arduino board.
This voltage regulator needs an input voltage that’s about 1.5V above its (regulated) 5V output that will feed the Arduino.
Arduino - note the recommended values of the Input Voltage (7-12V)
If you feed 5V into Vin, the voltage drop on the internal regulator will cause the Arduino board to be supplied with only about 3.5V, which is probably enough to make it function, but not enough to control the servos.
The solution, as you already found out, is to feed the 5V from the L298N board directly into the 5V pin of the Arduino. Please take care to measure the actual voltage from the L298N before you do this (it should be 5 Volts). Alternatively you could connect 12V to the Vin pin of the Arduino, but again, make sure you know what you’re doing.