Strangeness with ttyO5 RTS ?

Hi List,
i have a strange effect when using RTS for ttyO5.
I have the std. Overlay with ttyO5 enabled.
I set GPIO_11 direction=out, value=1 via /sys/class/....
in that moment RX on ttyO5 goes low causing spurious 0x00
on the serial line.

Why this ? Did i misunderstand the documentation ?