my wife complains about this a lot when she's behind a tesla and I see it all the time.
With our cars, we can gradually slow the car down within a short period without the brake lights kicking on. For example, you're at 40 mph and you gradually use the accelerator to decelerating to 35 mph.
can't really do that in an ICE without riding the brakes or let the car coast for awhile. if the person behind you is 1-2 car lengths behind, it's hard to visually see our cars slowing down since no brake lights are kicking on to alert them.
as a poster said earlier, there's that threshold before the brake lights kick on. the only way really to adjust this is have the brake lights come on with ANY slow down or maybe lower that threshold more. i dunno if that's a good solution either since it'll look like we're riding the brakes alot.