Asus RT-AC51U - problem adjusting transmit power

I yesterday recognized that the transmit power in my Asus RT-AC51U cannot be adjusted at all - it runs always at 100% and therefore it may exceed also the allowed levels here in Germany.

Does anyone have the same problem? I'am actually running the current stable release 18.06.1. I have measured the levels with some mobile devices and also with two other wireless routers (TP-Link Archer C7 and TL-WR1043). Whatever I set in the configuration I see no changes in any device,

Using the stock firmware of the Asus router the transmit power can be adjusted in a wide range.

Is this a problem of the Mediatek plattform?