I may have misunderstood this, or it might be device specific, but that's not my experience.
I have a Hue motion detector in my home office. In Home, I have automations set up so that if it detects movement, it will turn on my air purifier, and if it's after sunset, some lights. I have this automation set to turn off after 5 minutes.
Everything only turns off 5 minutes after I leave my office, other than that it will stay on for as long as I'm in the office. That suggests that whenever I move, the 'countdown clock' resets to 5 minutes, which doesn't seem to be what you're suggesting above?
I guess the behaviour may differ between different motion detectors. For example, some (typically in intruder alarm systems) will use battery-saving logic whereby if it detects motion, it won't 'look' again for another x minutes. If your 'off delay' was less than x minutes, you'd get the problem the OP mentioned.