I have seen two bars of charge twice in my HEV. Once was a very steep five mile climb at 25 mph (the five mile descent was the only time I've seen the battery charge graph full). The other was 40 miles highway in heavy rain.
The Niro can and does charge the battery without regen. Just watch the flow diagram, not to mention the battery charge level, while the engine is running and speed is constant (not slowing or going down a hill). As far as the PHEV goes, effectively the plug in energy is partitioned from the HEV energy, and under normal conditions the normal up and down charge running ICE in HEV mode does not affect that virtual (software) partition. However, you can charge the plug in partition by running Sport mode. Hard to imagine how that could benefit, running the ICE specifically to recharge the portion of the battery reserved for plug in is very inefficient.
2018 Kia LX HEV Metal Stream with Advanced Tech