I think hitting the power limit causes very very short declocks to very low speeds. When it triggers it goes to 300 MHz for xx microseconds, or something similar, so we see these randomly timed stops averaged into our normal max clocks. This causes the reported clock rate to fluctuate down but that does not tell us anything about the real min clock that would allow it to never throttle.
Also, Nvidia has done a pretty amazing job at keeping power down without hurting performance very much. Increasing the max clock keeps increasing performance, even when the card is already triggering the power limit. I like steady clocks but it looks like opening up the power a lot is usually worth less than a +50 MHz OC, unless you have an unusually power heavy load. On battery or heat constrained I think this throttling tech would actually be very good.
I am not sure about your auto-decreasing power limit, I never saw that with my FE 2080 Ti.