Welcome to Android Central forums. Congratulations on your new device. I had to look up a few things in your post to try and understand everything you're talking about.
First BGMI is an intense 3rd person shooter. Playing at the highest frame rate is going to have an impact on your battery percentage. More computing power = more battery drain. I also looked up your device to get a feel for your device. For a midgrade device that 5000+ mAh battery should give you some pretty good SOT, screen on time, and overall battery life.
I feel we need to take everything into consideration when we're having this discussion. We'll need a little more information to and let's keep in mind you're not charging your phone to 100%.
What percentage did you start at after removing from the charger? What percentage did you end at, or at least what was the percentage when you checked your stats? This is vital for us know what the true impact was in the numbers you're telling us about.
How long had it been since you unplugged? This gives us a percent pre hour number.
Disclaimer: Battery numbers don't work this way 1 for 1 but it should be ballpark.
To know what your numbers represent we can do some math. Let's say you started playing your game at 91%, right off the charger, and ended at 47% and you played for almost an hour, that's 91 minus 47 = 44. 40% of 44 is 17.6, which we'll round up to 18 and round your time up to 1 hour, and then we can say while playing you were using 18% battery per hour. While we really cannot extrapolate out to a 1:1 ratio we can assume that it would be close. Giving you 3.888 hours of play if you played, and did nothing else, from 90% to 20% battery level. That number came out larger than I expected, I may have been too generous saying 47% ending level, but this would be how you'd estimate the usage to get a feel for how long your battery might last. If it were 90% to 33%, and the game used 40% of the battery used, that would be 22.8, again we'll round up to 23% and one full hour. @ 23% per/hour that would give you 3 hours of game play from 90% to 20% if you were doing nothing else.
Tell us the actual numbers and we'll see what we come up with but I hope this helps you feel better.
Let us know what you find.