Why don't you just reproduce the setup and report back what you see? It's not that difficult and would be far more productive.
Because I do not have OP's signal available and also today I have not anymore Rigol 1000Z (what I know also well because I have owned one and used it)
But, what I see OP image and as far as I know, where ever he set trigger level there is aliasing due to low samplerate related to signal details. This is my opinion and based also to knowledge and lot of lab experience. I stay behind my words. If I get original OP's signal and this oscilloscope I proof it. Without it, I keep my opinion based to theory and mu experience.
Of course there can exist also other reasons and so on as you explain and I can well accept also these may happend and even exist in this case also. This do not disable clear aliasing what there is sure with 500MSa.
Trigger instability is other case. (I mean these visible acquisitions what are horizontally different location (randomly trigged) This may be in singnal under test but because in image it can clearly see what is in signal just around trigger level my suspect is that this disappear with small change in trig level.
But then other case.
Topic name is: Channel cross-talk issue on Rigol 1054Z ? (Rigol can here forget because it exist in all scope what I know, more or less.)
Attached: This is oscilloscope channels cross talk. (look V/div scales ratio (1:500) and estimate cross talk ratio)
CH3 Signal is 5V ~5ns pulses. CH4 open with BNC cap for reduce external EMI.