Siglent SDS1104XE: slanted square wave

Hi!

I've asked Siglent support but after a couple of responses the thread went cold. Maybe I'm being dumb but I've noticed that there's a frequency (low, around 100Hz) where scope response changes a bit. Below and above it square input looks square. Right about it square input looks slanted.

I tried to do a very slow "sweep" and there's small but visible change in the envelope. So, e.g. with a constant 600mV p2p input lower frequencies measure exactly that while higher ones measure 612mV, so ~2% diff.

Terminated 50Ohm cables (not that it matters at such a low freq) to be sure. Latest firmware, after full self-cal. Siggen itself seems allright, I have an ancient Tek scope and the siggen output looks Ok there with same input/same cables. Scope seems happy and fully functional otherwise, few years old though, out of warranty.

Has anyone else seen anything like that? Is this a normal behavior within the expected margin of error?

Thanks!