[NiLuJe]

Yep. That's how nightmode works (as in, it's terrible and I can't stand it).

Note that on the PW2+, that double-refresh is inhibited by the hardware, so it's slightly less terrible in that respect.

EDIT: Errm, wait, I might have misunderstood you. I was specifically thinking about how the black flash gets stuttery in nightmode on older devices. What you describe sounds worse than that.
On which FW version is that? The final release (5.6.1.1) might be slightly less awful, but maybe not. IIRC, the first significant nightmode enhancements came with FW 5.6.5, i.e., not on a PW1.