Well, the main cause of this is High temperature property and Thermal daemon mitigation.
What these two function do, is that they pump fown the FPS a touch to protect the phone from overheating and cool it down. It's not a bad idea but sometimes it may get a bit annoying when the power is really needed.
If you're a G3 user but don't have your phone rooted, nevermind! This time root access will not be needed.
- Access hidden menu on your G3.
To open hidden menu, open the dialer app input one of these numbers (that matches your carrier).
After you type the last digit, hidden menu will open on its own. If you're using Verizon carrier, you'll have to call the number.
3845#*855# (for International model)
3845#*851# (for T-Mobile)
3845#*850# (fot AT&T)
##228378 - Verizon
After you successfully opened the hidden menu, scroll down till you see High Temperature Property OFF and turn it ON. Enabling this will disable the high temperature property.
We're not done yet, scroll down a bit more till you see Daemon Mitigation OFF and again, enable it to turn this function off.
Now, reboot your G3. You're done.
Disabling these function will no longer turn the brightness down when the device gets overheated so beware when streaming or doing stuff with high brightness.