Hmm, maybe I am wrong, looking at one popular home improvement site, most of the toilets, including single flush, are listed as gravity-fed rather than siphon.

Hmm, maybe I am wrong, looking at one popular home improvement site, most of the toilets, including single flush, are listed as gravity-fed rather than siphon. So if the problem really is non-siphon toilets in general, it would apply to single and dual flush ones, at least the ones sold by this popular US retailer. (Lowes)