After some testing I found out it happens when Snap to Grid is enabled (which follows Quantize settings in my case, 1/16 usually). It doesn't respond to any configuration of Quantize / Timebase / Snap I tried. Also loop state & range doesn't seem to affect this, before anyone asks:
Snapping on:

However, with snapping off it works as expected:

Can someone with S1 3.x or 4.x check this, please?
I don't think this was happening before v5.1 but I'm not 100% sure, so I'd like to double-check before I file a bug ticket. I don't think it's intentional, it doesn't really make much sense and there's no word about it in User Manual