One technique that can help prevent these changes is by locking the ISO setting.
You can manually set the ISO value before recording to keep it constant throughout the recording. Additionally, using the manual exposure mode gives you more control over the camera settings. However, it's worth noting that exposure shifts in videos may not be a BM issue as mentioned. The automatic exposure setting may be adjusting the ISO value based on the lighting conditions in the scene.
Thus, turning off the automatic exposure setting can eliminate unexpected changes in your video's exposure.