Yes, this is normal. Because of noise, the gyro drifts. All gyros drift.
Form this page;https://ozzmaker.com/berryimu/
Gyros – A gyro measures the rate of rotation, which has to be tracked over time to calculate the current angle. This tracking causes the gyro to drift. However, gyros are good at measuring quick sharp movements.
Accelerometers – Accelerometers are used to sense both static (e.g. gravity) and dynamic (e.g. sudden starts/stops) acceleration. They don’t need to be tracked like a gyro and can measure the current angle at any given time. Accelerometers however are very noisy and are only useful for tracking angles over a long period of time.
Then further down the page;
Once you have the both the angles from both sensors, you will have to combined them to overcome the gyro drift and the accelerometer noise.
Once the values are combined, you should have a perfect angle reading.
Mark --OzzMaker.com --