I think this might be same thing that is being pushed recently for all browsers and I described it in:
There is a way of turning off manually Tab Discards , but no way to turn off Tab Freeze.
Feel free to check my topic for explanation, but I do not know of any browser option to turn it off completely. This was just pushed and we can’t opt it out.
In short:
“Looking through similar issues I found out about brave://discards/ but those settings reset every time and do not allow me to turn off this option completely.”