Cause:

This issue will occur if the flash is activated only once when you start taking burst shots in Burst shoot mode. In this case, your phone will deem there to be sufficient light for the first few photos and shorten the exposure time for them, which decreases the brightness of these photos. Once the flash turns off, the exposure time for subsequent photos will match the dark environment and their brightness will return to normal.

Solution:

Before using Burst shoot mode in poorly lit conditions, select the flash to be always on ().