I’m not saying it’s not normal, but it’s just not desirable nor good. And there’s solutions to solve this issue, which are good lipos or higher p li-ions. @Jinra and @chaka are better people than me to talk about the details of voltage sag.
What lipos are you using btw?
The best lipos I used were the zippy flightmax 8000 mAh cells, and they sagged very little.